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What Are the Aims of Science?

What ue lhe llbas of Science?



If we are to understand the nature of science,
·”course much wider than science. We all, including
we must see it as an activity and achievement of
infants and children, aim to extend our knowledge
the human mind alongside others, such as the
a~d understanding: science is unique only in the
achievements of children in learning to talk and
rigour, system, and amount of co-operation between
to cope with people and other objects in their
individuals involved in its methods. For the preenvironment, and the achievements of non-scientsent, however, I shall not explore the peculiarities
ists living in a rich and complex world which
of science, since what it has in common with other
constantly poses problems to be solved. Looking
forms of acquisition of knowledge has been too long
at scientific knowledge as one form of human knowneglected, and it is the common features I want to
ledge, scientific understanding as one form of

human understanding, scientific investigation as
In particular, notice that one cannot have the
one form of human problem-solving activity, we can
aim of extending one’s knowledge unless one prebegin to see more clearly what science is, and also supposes that one’s knowledge is incomplete, or
what kind of mechanism the human mind is. I
perhaps even includes mistakes. This means that
suggest that no simple slogan or definition, such
pursuing the aim requires systematic se~f-criticism
as can be found in textbooks of science or philoso- in order to find the gaps and errors. This distinphy can capture its aims. Science is a complex
guishes both science and perhaps the curiosity of
network of different interlocking activities with
young children from some other b~lief systems,
multiple practical and theoretical aims and a great such as dogmatic theological systems and political
variety of methods. I shall try to describe some
ideologies. But it does not distinguish science
of the aims and their relationships in this essay.

from philosophy.

, OVersimple characterisations, by both scientists
and philosophers, have led to unnecessary and
A further subdivision: form and content
crippling restrictions on the activities of some
The factual aim, extending knowledge and underwould-be scientists, especially in the social and
standing, can be further subdivided as follows:

behavioural sciences, and to harmfully rigid
l.a Extending knowledge of what sorts of things
barriers between science and philosophy.

are possible and impossible in the world,
By undermining the slogan that science is the
and how or why they are (the aim of interpretsearch for laws, and subsidiary slogans such as
ing the world, or learning about its form)
that quantification is essential, that scientific
knowledge of what particular objects,
theories must be empirically refutable, and that
or states of affairs exist
the methods of philosophers cannot serve the aims
or existed in particular places at particular’

of scientists, I shall try, in what follows, to
times (the aim of acquiring ‘historical’ knowliberate some scientists from the dogmas indoctrinledge, or learning about the contents of the
ated in universities and colleges. I shall also

try to show philosophers how they can contribute to
the scientific study of man, thereby escaping from
A similar distinction pervades the writings of
the barrenness and triviality complained of so often Karl Popper, though he would disagree with some of
by non-philosophers and philosophy students.

the things I say below about (l.a). Different
branches of science tend to stress one or other of
A side-effect which will be re~orted elsewhere,
is to undermine some old philosophical distinctions these aims, though both aims are usually present to
and pour cold water on’battles which rage around
some extent. For instance, physics is lOOre conthem – like the distinction between subjectivity
cerned with aim (l.a), whereas astronomy is perhaps
and objectivity, and the battles between empiricists more concerned with (l.b). Geology, geography,
and rationalists.

biology, anthropology, human history, sociology, and
some kinds of lingUistics tend to be more concerned
First crude subdivision of aims of science
with (l.b), i.e. with learning about the particular
contents of particular parts of the universe.

Science has not just one aim but several. The aims
Chemistry, some branches of biology, economics and
of scientific investigation can be crudely subpsychology attempt to investigate truths not so
divided as follows:

restricted in scope. In the jargon ot philosophers,
1 to extend man’s knowledge and understanding of
(l.a) is concerned with universals, (l.b.) with
the form and contents of the universe (factual

aims) ,
However, the two scientific aims are very closely
2 to extend man’s control over the universe, and
linked. One cannot discover what sorts of things
to use this to improve the world (tec9nological
are possible, nor test explanatory theories, except
aims) ,
by discovering particular facts about what actually
3 to discover how things ought to be, what sorts
exists or occurs. Conversely, one cannot really
of things are good or bad and how best to further
understand particular objects, events, processes
the purposes of nature or God (normative aims).

Some of the work on this paper was done during
Whether the third aim make s sense (and many
tenure of a visiting fellowship at the School of
scientists and philosophers would dispute this)
Artificial Intelligence, Edinburgh University.

depends on whether it is possible to derive values
aQd norms from facts. I shall not discuss it as it I am grateful to the Science Research Council and
Professor Bernard Meltzer for making this possible.

is not relevant to the main purposes of this’ enSeveral colleagues have helped me by criticising
quiry. The second kind of aim will not be given
much attention either, except when relevant to dis- drafts of parts of this paper. P.M. Williams,
L.A. Hollings and G.J. Krige in particular wrote
cussions of the first kind of aim.

Th:e first kind of aim, like the others, is of
at some length about my mistakes and omissions.



etc, except insofar as one classifies and explains
them in the light of lOOre general knowledge about
what kinds of things there can be and how or why.

These two aims are closely linked in all forms of
learning about the world, not only in science.

Notice that I have characterised these aims in a
dynamdc form: the aim is to extend knowledge, to go
on learning. Some might say that the aim is to
arrive at some terminal state when everything is
known about the form and content of the world, or
at least the form. There are serious problems about
whether this suggestion makes sense: for example,
how could one tell that this goal had been reached?

But I do not wish to pursue the matter. For th~
present, it is sufficient to note that it makes
sense to talk of extending knowledge, that is removing errors and filling gaps, whether or not any
final state of complete knowledge is possible.

Some of the criteria for deciding what is an extension or improvement will be mentioned later.

Many philosophers of science have found it hard
to explain the sense in which science makes progress, or is cumulative. (e.g. Kuhn, 1962, last
chapter). This is because they tend to think of
science as being mainly concerned with laws; and
supposed laws are constantly being refuted or replaced by others. Very little seems to survive.

‘But if we see science as being also concerned with
knowledge of what is possible, then it is obviously
cumulative. For a single instance demonstrates a
new possibility and unlike a law this cannot be
cefuted by new discoveries, even if the possibility
is re-described from time to time as the language
of scientists evolves. Hypotheses about the limits
of these possibilities (laws) lack this security,
for they are constantly subject to revision as
the boundaries are pushed further out, by newly
discovered (or created) possibilities. Explanations
of possibilities and their limits constantly have
to be refined or replaced, for the same reason.

But this is all a necessary part of the process of
learning and understanding more abo,ut what is
possible in the world. It is an organic, principled
growth, even if people do sometimes disagree about
what is and what is not progress, for reasons to
be described later.

Let us now look more closely at aim (l.a), the aim
of extending knowledge of the form of the world.

merely conceivable or representable but really can
exist or occur. Finding out what aotually exists,
and trying to make new things exist, are often
means to th,is end. We oan distinguish knowledge
of absolute possibility concerning a phenomenon X
(X can exist) from knowledge of relative possibility (X can exist in conditions C). Extending
knowledge of relative possibilities for X is an
important way of extending knowledge of what is
possible. All this should be distinguished from
the aim (e) below, of finding out what kinds of
things are most likely, common or frequent, either
absolutely or in specified conditions. The latter
is a concern with probabilities not possibilities.

Aim (b) clearly presupposes aim (a), for one can
only acknowledge possibilities that one can conceive of, describe or represent.

(c) Constructing theories to explain known possibilities: i.e. theories about the underlying
structures, mechanisms, and processes capable of
generating such possibilities. For instance, a
theory of the constituents of atoms may explain
the possibility of chemical elements with different
properties. ‘How is this possible?’ is the typical
form of a request for this kind of explanatory
theory, and should be contrasted with the question
‘Why is this so?’ or ‘Why is this impossible?’,
discussed in (f) below.

(d) Finding limitations on combinations of known
possibilities. These are often called laws of
nature: for instance to say that it is a law of
nature that all X’s are yts is to say that it is
impossible for something to be both an X and not a
Y. It is these laws, limitations or impossibilities
which make the world relatively stable and predictable. This aim, like (c), presupposes aim (b), since
one can only discover limitations of possibilities
if one already knows about those possibilities.

(This aim of science is the one most commonly
stressed in the writings of scientists and philo-‘

sophers. It subsumes the aim of discovering causal
connections, since X causes Y if the occurrence of
X makes the non-occurrence of Y impossible.)

(e) Finding regular or statistical correlations
between different possibilities, for instance
correlations of the form ‘In conditions C, 90% of
all X’s are yts’. This is a search for probabilities. It presupposes aim (b) for the same reason
The interpretative aims of science
as (d) does. Except in quantum physics, ~he
search for such statistical correlations is really
The aims listed below together constitute the aim
only a stopgap or means towards aoquiring a deeper
(l.a) of interpreting the world, or learning ,about
its form. They are all so closely related that’ to
understanding of the sort described in (d) above.

treat them as separate aims would be artificial.

Alternatively, it may be an aim of a historical
Similarly, to call some of them ‘scientific’ and
science: facts about relative frequencies and proothers ‘metaphysical’ or ‘philosophical’, as empiri- portions of various kinds of objects, events or
cists and Popperians tend to do, is to ignore their processes are often important facts about the
interdependence. Rather,’ they are all aspects of
content of a particular part of the world. For
one aim. For convenience I shall talk of them as
instance, most of the correlations unearthed by
separate aims, but this will be qualified by dessocial ,scientists are culture-relative. SUch
cribing their connections. They are:

information may have practical value despite its
theoretical poverty.

(a) Development of new concepts and sgmbolisms
making it possible to conceive of, represent, think (f) Constructing theories to explain known impossibilities, laws and correla~ions. SUch theories
about and ask questions about new kinds or ranges
of possibilities (e’.g. new kinds of physical subanswer ‘Why?’ questions, and are generally refinestances, events, processes, animals, mental states, ments of the theories described in (c). ‘That is,
explaining limits of possibilities (i.e. explaini~g
human behaviour, languages, social systems, etc).

laws) presupposes or refines an explanation of the
This aim includes the construction of taxonomies,
possibilities limited. The theory of molecules
typologies, scales of measurement and notations
for structural descriptions. This extension of our composed of atoms which can recombine explains the
possibility of chemical change. Further refineconceptual and symbolic powers is one of the major
ments concerning weights and valencies of atoms
functions of mathematics in science.

explain the observed limitations: the laws of con(b) Extending knowledge of what kinds of things
stant and multiple proportions.

(including events and processes) are possible in
the warid; i . e •. wha,t kinds of things axe not
(g) Finding and eJJ;minating inade~te concepts,


<ltJ '.

possibility refutes the theory that water is a

chemical element and corroborates the alternative
hypothesis that all water is composed of hydrogen
and oxygen, and also more general theories about
possible kinds of transformations of matter. Similarly, although an ‘historical’ biologist may be
interested in recording, for a fascinated public,
the flora and fauna of a foreign isle, or the antics
particularly intelligent chimpanzee, the ‘int’erpretative’ biologist is interested only insofar
as they illustrate something, such as what kinds
of plants and animals can exist (or can exist in
certain condi tions) , or’ what kinds of behaviour are
possible for a chimpanzee I or for some other class
containing the ,animal in question.

In short, the interpretative scientist studies
the form of the world, using the contents only as
evidence, whereas the historical scientist simply
studies the contents.

There is, of course, no reason. why anyone science, or scientist, should be classified entirely
as interpretative, or entirely as historical.

Different elements may intermingle in one branch
of science. For instance, a linguist studying a
particular dialect is an int,erpretative scientist
insofar as he is not concerned merely to record the
Newcomen’s beam engine
actual set of sentences uttered by certain speakers
of that dialect, but to characterise the full range
syIDbolisms, and laws”about what is and is not
of sentences that would or could be intelligible
poss~ble, and explanations of possibilities and
to an ordinary speaker of that dialect, namely, a
laws. That this is an aim of science is, as
range of possibilities. However, insofar as he is
already remarked, implied by saying that an aim
interested merely in finding out exactly what diaof science is to extend knowledge. As many philolect is intelligible to a certain spatio-temporally
sophers of science have po.inted out, it is not
restricted group of persons, he is an historical
generally possible to prove explanatory theories
linguist, as contras~ed with a linguist who is
in science: at most they can only be refuted or
interested in this dialect primarily as a sample
shown to be inadequate in some way. Moreover,
of the kinds of language which human societies can
when several candidates survive refutation, the
develop: the attempt to characterise this set of
most that can be done is to compare their relative
possible languages is often called the search for
merits and faults, without necessarily establishing linguistic universals.

the absolute superiority of one over the other.

Thus a richer philosophical terminology would be
required for a precise description of hybrid histIt is often assumed that the only kinds of proper
orical and interpretative aims. This is not reletests are empirical (i.e. observations of new.

facts, in experiments or in nature). However, we
vant to our present concerns and will not be
shall see that many important tests are not
pursued further. Instead, in II-IV below, I will
empirical. We shall also see that just as negative concentrate mainly upon an analysis of the first
instances count against laws, so do positive inthree components of the interpretative aim, outlined
stances provide support for theories about possiabove. These three are very tightly interconnected.


It is very hard to describe the distinctions between
If forced to summarise all this in a single
them accurately, and I am sure I do not yet underslogan, one could say:

stand these matters aright. Moreover, each of them
A major aim of science is to find out what spits
could be further subdivided. Detailed historical
of things are and are not possible in the world I
analysis is required here,’ so that similarities and
and to explain how and why.

differences between cases can be described accurateThough too short to be clear, this may be a useful
ly and a more satisfactory typology developed; a
antidote to more common slogans stressing the discontribution to the scientific study of science.

covery and explanation of laws and regularities.

Alas, this will require the help of persons more
SUch slogans lead to an excessive concern with
scholarly than I.

prediction, control and testing, topics mainly
~elevant to aims (d) to (g), while insufficient
attent~on is paid to the more fundamental aims
(a) to (c), especially in psychology and social
science. The result is sloppy research, theorisIndividuals (and cultural groups) can differ not
ing and teaching.

only in the things they know or believe, but also
More about interpretative and historical aims.of
in the possibilities they can grasp, the concepts
they have, the generative power of the languages
they use, the questions they can ask.

Unlike the historical scientist, the interpretative
As new conc1epts and symbolisms are developed,
scientist is interested in actual objects, events
and the language extended, new questions become
or situations only insofar as they are specimens of askable. For instance, people who grasp the conwhat is possible. The research chemist is not
cepts ‘hotter’ and ‘longer’ can understand the
interested in the fact that this particular sample
question whether metal rods get longer when they
‘of water was, on a certain day, decomposed into,
are made hotter. And they may even be able to grasp
hydrogen and oxygen in that laboratory,’ except
crude distinctions between metals according to which
insofar as,this illustrates something universal,
grows longer faster when heated. But in order to
such as the possibility of decomposing water. This learn to think about whether the change in length


The Role of Concepts and Symbolisms


is proportional to the change in temperature, so
interpreting evidence and ass~ssing hypotheses.

that they can then use the constant of’proportionOur minds may use sy.mbo1isms which we can only
a1ity (divided by the length of the rod) to define
translate into actions, not spoken or written lana numerical ‘coefficient of ~ansion’ for each
guage – until we have extended our language. Nonmetal, they need to grasp numerical representation
logicians can often distinguish valid from invalid
of differences in temperature and length (‘hotter
arguments without being able to say how. They have
by how much?’, ‘longer by how much?’). Similarly,
not learnt the overt language of logicians.

although people may have a crude grasp of distincEven after going a stage further and learning how
tions between velocity and acceleration, and be
to articulate their reasons, scientists may not yet
able to detect gross changes in either, on the
have learned’ how to teach their new concepts to
basis of their own experiences of moving things,
colleagues and rival theorists. So attempts at
being moved, and perceiving moving objects, neverrational persuasion break down. This has misled
theless, until they have learnt how to relate consome philosophers and historians of science into
cepts of distance and time to numerical interval
thinking that there are no reasons, so that the
scales, they cannot easily make precise distincdecisions of scientists are irrational or nontions between different velocities, or between
rational. This is as silly as assuming that a
acceleration and rate of change of acceleration,
mathematician is irrationa1y simply because he
nor think of precise relations between these
cannot explain a theorem to a four year old child.

The child may have much to learn before he can

understand the problem, let alone the reasoning,
These familiar examples show the power of extendand the mathematician may be a poor teacher.

ing scientific language by introducing numerical
Concepts are not simple things which you either
concepts and notations corresponding to old nongrasp or don’t grasp, or which can be completely
numerical concepts. This sort of thing has been so
conveyed by an explicit definition or axiomatic
important in physics that many biological and socharacterisation. For “instance, as work of Piaget
cial scientists have been deluded into thinking it
has shown so clearly, and Wittgenstein less clearly,
part of the definition of a scientist that he uses
many of our familiar concepts, like ‘number’,

‘more’, ’cause’, ‘moral’, and ‘language’, are very
The replacement of Roman numerals with the Arabic
complex structures of which different fragments
system is an example of a powerful notational admay be grasped at different times. A child aged
vance. Another was the Cartesian method of using
four or five may be able to count flawlessly well
arithmetic to represent geometry and vice versa.

beyond 20, and use counting to get correct answers
However, non-numerical conceptual and notational
to simple addition problems, yet be quite unable to
devices have also ‘been important, such as concepts
used in describing structures of plants and anima.1s, count backward”s, or to answer questions like ‘What
comes before 6?’, ‘Is 9 after (bigger than) 5?’.

concepts used for describing structures of mechan(Yet he succeeds with the numbers on a clock in
ica1,systems and electrical circuits (geometrical
and topological concepts), taxonomies or typo10gies, front of him, so he understands the questions.)
Does he or does he not understand the concept of
and granmatica1 concepts (see N. Chomsky, 1957).

number, or of 6? His failure does not prove that
All sorts of notations besides numerical and algebraic ones have played an important role in extend- he is irrational: he still has some procedures to
invent for himself – if his parents and teachers
ing the abilities of scientists to express what
don’t destroy his creative abilities.

they know and want to find out, for instance picThe more of one’s concepts and associated protures, diagrams, maps, models, graphs, flow charts,
cedures one is able to represent explicitly in
and non-numerical computer programs. Examples
symbols of some sort, the greater one’s power to
include the diagrams used in the study of levers,
explore possibilities systematically by manipulatpulleys, bending beams, and other mechanical sysing those symbols. For instance, by explicitly
tems, the ‘pictures’ of molecules used by chemists,
characterising aspects of our intuitive grasp of
for instance in the following representation of
spatial structures in the form of axioms and definithe formation of water from hydrogen and oxygen
tions, one becomes able to experiment with altera( H-H, H-H, 0-0) +
(H-O-H, H-O-H )
tions in the axioms and definitions, and thereby
circuit diagrams used in electronics, optical draw- invent concepts of non-euc1idean or other new sorts
ings showing the pa fus of light-rays, plates showof geometries. In this way one can learn to think
ing tracks of subatomic partic1es,and the ‘trees’

about new sorts of possibilities without waiting
used to represent deep-structures of sentences.

to be confr9nted with them.

(This kind of thing
Concepts may also be used without being represen- may also happen below the level of. consciousness,
ted explicitly by any external symbol. There are
in children and scientists, as part of the process
philosophers who dispute that these are cases of
of learning and discovery.) Of course, one may
the use of concepts, but in the face of well known
also extrapolate too far, and construct representfacts I can only regard this as verbal quibbling.

ations of things which are not really possible in
the world, so empirical investigation of some sort
We know that young children and other animals can
discriminate, recognise and react intelligently to
is required to discover whether things which are
conceivable or representable can also exist. For
things which they cannot name or describe. The
instance, merely analysing the concept of an
consistency and appropriateness of their behaviour
shows that they act on the basis of reasons, even
element with atomic number 325 will not decide
whether such a thing can occur. This is the
if they cannot articulate them or are unaware of
reason for distinguishing the first aim of interthem. The same is true of an adult who cannot
describe the features of musical compositions which pretative science, namely extending concepts and,symbo1isms, from the second aim, namely extending
enable him to recognise styles of composers and
knowledge of what is really possible. (I believe
appreciate their music, or the cues whic.h enable
him to judge another’s mood. No doubt this is true many of these ideas are to be found in Kant (1781».

also of many scientists, especially when they are.

Two phases in knowledge-acquisition: understanding
in the early phases of some kind of conceptual
development. They may then, like children and chim- and knowing
panzees, be unable to articu1a~e fully the reasons
It is not always noted in epistemological discus~ey have for some of the decisions they take about

sions that there are two important phases or steps
vance to a variety of new concrete problems, e.g.

in the acquisiti~n of ,knowledge. Discovering that
going from understanding algebra’to being able to
p is true first of all requires the ability to
apply it in real life.

(There are many other _
understand the possibility that p might be true and cases not so closely linked to science, e.g. the
might be false, which requires grasping the congrowth of social, moral and political consciouscepts used in the proposition p. The s~cond phase
ness. Learning to feel shame, embarrassment, or
is finding out that p is true, for instance by
guilt, as contrasted with annoyance or regret,
empirical observation, use of testimony, inference
requires complex cognitive development, and the
from what is already known, or some combination of·, same is true of many other human emotions. Some
these. In the first phase one is able to ask a
I”’concepts e. g. ‘impertinent’ are on:!-y intelligible
question, in the second one has an· answer.

in certain cultures.)
may be primitive kinds of knowledge-acquisition in
Until these conceptual changes are better underwhich questions are never understood, only informastood, discussion,of ‘incommensurability’ of
tion acquired and used. But science is not like
scientific theories and of the role of rationality

in science is premature. Meanwhile education will
Usually philosophers plunge into discussions of
continue to be largely a hig and miss affair, with
such questions as whether we can know anything about teachers not knowing what tl).ey are doing or how
the future, or rationally believe anything ,about the it works.

future, without first asking how a rational being
To sum up so far. A system of concepts and
can even think about the future or think about alsymbols constitutes a language.

(This statement
ternative possible future states of affairs. They
is grossly inaccurate, but will do for present
are therefore attempting to assess the rationality
purposes.) A language which is used to formulate
of certain decisions on the basis of a drastically
one theory will usually also contain resources for
incomplete account of the resources that might
formulating alternatives, including the negation
enter into the decision-making process. The reason of the theory and versions of .the theory in which
why this has been shirked is partly because it is
some predicate, relational expression or numerical
so hard to do, partly because of an unwarranted
constant is replaced by anoth~r •. So concepts and
restriction of rationality to relations between
symbols are tools for generating possibilities or
evidence and belief-contents, and partly because
questions for investigation. They have greater
many philosophers (unlike Wittgenstein, 1956),
generative power than theories. The scientist who
think that the investigation of conceptual mechan- I usefully extends the language of science, unlike
isms is a task for psychologists not philosophers.

‘one who simply proposes a new theory using existHowever, most psychologists never even think of
ing concepts and symbols, extends the hypothes+sthe important questiohs, ang those who do usually
forming powers of the scientists who understand him.

lack the techniques of conceptual analysis required
In this sense conceptual advances are more profound.

for teaching them: so the job does not get done.

The important differences between modern scient(Piaget seems to be an exception. But his followists and those of the distant past therefore coners seem capable only of repeating his experiments,
cern not merely the statements and theories:thought
and not of extendi~g his conceptual analyses.)
to be true or f~lse, but also which statements and
There is a need for a tremendous amount of retheories could be thought of at,all. Not only are
search into what it is to understand various sorts
more answers known now, but more questions are
of concepts, and what makes it possible. There is

also a need for some kind o~ t~xo~o~y of types of
Criticisin conce tual systems
conceptual change, whether ~n ~nd~v~duals or cultures.

The efforts made so far by psychologists to produce
Sometimes old questions become unaskable as a resuch taxonomies capture only a tiny fragment of the
suIt of conceptual change, like questions about
range of possible developments. Here are some
phlogiston or absolute velocity, or perhaps ‘mediexamples of possibilities of conceptual change
,cal’ questions like ‘What did he do to deserve
which still require adequate explanations. Going
this affliction?’ Modern medical science contains
from grasping a relation like ‘hotter’ or ‘longer’

no means of generating possibilities constituting
to grasping that it can be used to define equivalanswers to this question, though both laymen and
ence classes of objects of the same temperature or
some medical men (on Sundays?) may still formulate
length. Going from this to grasping the possibility them.

(Incompatible systems of concepts and theoof cdmparing differences in temperature or length
ries may coexist in one mind – but that’s another
(i.e. understanding an interval scale). Going from
story.) So science is served not only by extendgrasping some general concept defined in terms of
ing and differentiating existing concepts: rejeca structure, or a function, or some combination of
tion of a concept or typology or mode of representstructure and function, to grasping systematic
ation may also serve the aims of science by reducprinciples for subdividing that concept into differ- ing the variety of dead-end questions and theories.

ent categories. Learning to separate the structural Concepts, typologies, taxonomies, and symbolisms
and functional aspects of a hybrid concept, like
can, like theories, be rationally criticised, and
‘knife’, or ‘experiment’. Changing a concept by
rejected or modified.

changing the theories in which it is embedded, in
There are several ways in which a typology and
the way that the concept of mass was changed by
associated notation can be criticised. For instance
going from Newtonian mechanics to Einstein’s mechone may be able to show
anics. Developing a new more powerful symbolism
“(a) that there are some possibilities it doesn’t
for an old set of concepts: e.g. inventing differallow for,
ential calculus notation for representing changes,
(b) that it represents as possible some cases which
or qsing the concept of a mathematical function
are not really possible,
to generalise earlier concepts of regularity of
(c) that some of the subdivisions it makes are of
correlation. Coming to see something in common
no theoretical importance,
between things one has never previously classified
(d) that some category within it should be subtogether, like mass and energy, particles and
divided into two or more categories, because
waves, straight lines and geodesics on a sphere.

their instances have different relations to the
Going from knowing a set of formulae and how to
other categories,
manipulate them to being able to see their rele{e) that the principle of subdivi’sion is too vague



to decide all known cases,
(f) that the classification procedure generates inconsistent classifications for some instances,
(g) that the notation used does not adequately
reflect the structural properties ofwthe typo~
logy, or of the instances,
(h) that the concepts used generate questions which
apparently cannot be answered by scientific
investigation (like the question ‘How fast is
the Earth moving through the ether?’) I
(i) that more powerful explanatory. theories can be
developed using other tools for representing

(It may be that some of these criteria are used,
unconsciously of course, not only by scientists,
but also by young children in developing their
conceptual systems.)
Several of these criteria will remain rather obscure until later. In particular, the first two
can only be understood on the basis of a distinction between what is conceivable 9r representable
and what is really possible in the world. We now
examine this, in order to explain the difference
between the first two interpretative aims of

Conceivable or Representable
versus Really Possible
The second interpretative aim of science is to
find out what kinds of things really are possible
in the world and not merely conceivable. This
includes such aims as finding out ‘what sorts of
physical substances, what kinds of transformations
of energy, what kinds of chemical reactions, what
kinds of astronomical objects and processes, what
kinds of plants and animals, what kinds of animal
behaviour, what kinds of mental development, what
kinds of mental abnormality, what kinds of language
and what kinds 6f social changes can exist or occur.

This aim is indefinitely extensible: having found
out that X’s can exist or occur, one can then try
to find out whether X’s can exist or occur in
specified conditions Cl, C2′ C3′”
having found that. objects can have one range of
properties which can change (e.g. length) and can
also have another range of properties which can
change (e.g. temperature) one can then try to find
out whether these properties can change independently or each other in the same object, such as a bar
of metal, or a particular object in specified circumstances, such as a bar of metal under constant
pressure ot tension. SUch further exploration of
the limits of combinations of .~ow.n possibilities
merges into the search for laws· and regularities,
as explained previously.

We can conceive of, or describe, a ‘lump of wood
turning spontaneously into gold, or a human living
unclothed in a vacuum, but it does not follow that
these things really can exist. What is the difference? First we look at what it is for something
to be conceivable, representable, or describable.

As philosophers well know, the subjective
of intelligibility, the feeling of having understood or imagined something, is no guarantee that
anything consistent was understood, imagined or
conceived of. If someone claims to be able to
conceive of the set of all sets which do not contain themselves, then provided he is using words in
the normai way we can show, by Russell’s well known
argument, using steps that he will accept if he is
reasonable, that he was wrong, or that his ‘conceiving’ amounted to nOL~ing more than repeating
the phrase, or some equivalent, to himself. A


sentence,· phrase, picture, diagram, or other complex symbol will, if intelligible, be part of a
language which includes syntactic· and semantic
rules in accordance with which the symbol is to be
interpreted. The mere fact that the symbol is
syntactically well-formed does not guarantee that
it can be interpreted, though it may mi’slead us
into thinking it can. More precisely, it may have
a sense but necessarily fail to have any denotation.

Thus the question ‘Does the table exist more slowly
than the chair?’ is syntactically perfect but we
can show that so long as the words are used according to normal semantic rules there can be no answer to the question. For f ‘more slowly’ when qualifying a verb requires that v;rb to denote a process or sequence involving changes other than the
,ctlange of time, so that the rate of change or
succession can be measured against time. Existence
is not such a process, so rates of existence cannot be compared. (For more on this see Sloman,
1971b. For non-verbal examples see Clowes, 1971)
So we can use the notion of what is coherently
describable or representable in some well defined
language or representational system, as an objective semantic notion. What is conceivable to a
person, will be what is coherently representable
in some symbolic system which he uses, not necessarily fully consciously. It may be very hard,
even for him, to articulate the system he uses,
but that does not disprove its existence. These
notions are as objective as the notion of logicai
consistency, which is a special case. However the
mere fact that something is, in this sense, representable or conceivable does not mean that it
really ,can exist. Conversely, what can exist ne8;d
not be representable or conceivaBle using the
symbolic resources available to scientists (or
others) at any particular time: their language may
need to be extended. Scientists (like children)
may be confronted with an instance of some possibility, like inertial motion, diffraction, or curvature of space-time, without seeing it as such
because they lack the concepts. (Kuhn, 1962,
chapter X, has overdramatised this by saying they
inhabit a different world.)
The word ‘possible! as I have used it, and as
others use it, tends to slide between the two cases
(a) used as a synonym for ‘consistently representable or describable using some language’, as in
‘logically possible’, and ~) used to refer to what
can occur or exist in the world. But what is the
difference between (a) and (b)?

This is not an easy question to,answer completely.

The main difference is that ‘conceivability or representability can be established simply by analysing the sentence or other symbol used and checking that the syntactic and semantic rules of the
language in question do not rule out a consistent
interpretation (which is not always easy), whereas checking whether something really is or is not
possible requires empirical investigation of some
sort. If an actual example. is found, that conclusively establishes the ‘possibility. The corresponding kind of impossibility is very much harder
to establish, and perhaps can never be conclusively established, though one can often be fairly
sure that something is not possible in. the world
either because of extensive and varied attempts
to realise it, or on the basis of inference from
some well established theory. (For instance, I am
convinced by physical and biological arguments that
it is impossible for a human being to live without
clothing in a vacuum.)
However, possibility is not the same as actual
existence. To say that it is possible for ten

in the world. It is by no means clear that such a’

drugged ailigators to be painted with red and
yellow stripes and then piled on top of one another, presupposition is intelligible. Moreover as a
definition it introduces a circularity, since it
is not to say that this ever has happened or ·will
is notoriously hard to define the concept of a
happen. Similarly, to say that several courses of
law without presupposing the concept of possibility
action are possible for me, is not to say that I
shall actually follow all of them. So, in saying
or same related concept.

Despite the remaining obscurities, I hope I have
that one of the aims of interpretative science is
done enough to indicate both ,that the first two
to find out which kinds of things are possible in
aims of interpretative science are different, and
the world, I do not mean that the aim is to find ~.

.” also tilat they are very closely related. Now for
out which kinds actually exist, as in historical
a closer look at the third aim – the aim of exscience. The latter is just a means, since existplaining possibilities. I feel least satisfied
ence establiShes possibility. (See part one for
wi th what I have to say about this.

more on this.)
What other means are there of deciding that something is really possible, besides finding an instance? Alas, the only answer I can give to this
is that we can reasonably, though always tentatively
perhaps, infer that something is possible if we
have an explanation of its possibility. What ,this
amounts to is roughly the following: (a) we can
consistently represent it using symbolic resources
which have already been Shown to be useful in
representing what is actual, and (b) it is not
ruled out by any well established law or theory
specifying limitations on possibilities. Perhaps
an extra condition is required: (c) it should
differ from already realised possibilities only/in
ways which are in some sense well understooQ. However, it is not clear that (c) adds anything to (a)
and (b). It is clear that these conditions do not
conclusively prove something to be possible, for
they rest on current theories,· of the limitations
of what is possible and such theories being empirical are gound to include errors and omissions,
at any stage in the advance of science. Further,
these conditions do not yield clear decisions in
all cases. For instance, is it reasonable to believe that it is possible for a normal human being
to be trained (perhaps starting from birth) to run
A request for an explanation of a possibility or
a mile in three minutes? It may not be clear
range of possibilities ~s characteristically exwhether we already know enough to settle such a
pressed in the form ‘How is X possible?’ Unfortquestion.

unately, the study of the role of such explanations
The above conditions for proving unrealised
in our thought is made more difficult by the fact
possibilities need to be further defined and
that not everyone who requires, seeks or finds
illustrated. For the present, however, my aim is
such an explanation, or who learns one from other
simply to indicate roughly how something can be
people, asks this sort of question explicitly, or
shown to be possible without producing an instance. fully articulates the explanation when he has underSo I have demonstrated that possibility is a
stoqd it. This partially explains why the role of
different concept from conceivability (or coherent possibilities and their explanations in science
representability), and also different from existhas noe been widely acknowledged.

ence. But I still have not given anything approxRoughly, an explanation of a possibility or range
!mating to a complete analysis: this would require
of possibilities can be defined ‘to be some theory
very much more than describing the criteria for
or system of representation which generates the
deciding whether something is possible or not.

possibility or set of possibilities, or rather
It would also require analysis of the role of the
representations or descriptions thereof. Even more
concept of possibility in our thinking, problembriefly, an explanation of a range of possibilities
solving, deliberating, regretting, blaming, praisis a grammar for those possibilities. There is
ing, etc, and its relations to a whole family of
much to be clarified in these formulations, but
modal words, such as ‘may’, ‘can’, ‘might’, ‘could’, first some examples from the history of science.

‘would’, etc. A mammoth task. (For some useful
Examples of theories purporting to explain
beginnings see Gibbs, 1970).

At any rate, we cannot analyse ‘Things of type
X are possible’ as synonymous with ‘Either things
of type X already exist, or else they are consist- The examples which follow are not all correct exently representable in our symbolic system without planations. Some have already been superseded and
others probably will be. The ancient theory of
being ruled out by known laws’, since this would
epicycles used geometry to explain how it was
define real possibility in terms of the current
possible for the apparent paths of planets to exsystem of concepts and~eliefs. We could try a
formula like ‘Things of type X are possible if and hibit irregularities while the actual paths were
constructed out of regular circular motions. Known
only if they either exist or are consistently
forms of motion were compounded in a representarepresentable in some useful representational
tion of new ones. The atomic theory after Dalton
system and are not ruled out by any true laws’.

But this has the disadvantage· of presupposing that explained how various kinds of chemical transformathere exists some complete set of true laws formu- tions were possible without any change in basic
lated in some unspecified language which correctly substances. (It also explained why the range of
possihi 1 i t i e s was restricted aocording to the laws
defines all the l~tations on what is possible

Explanations of Possibilities



‘of constant and multiple proportions, so that it
was vastly superior to previous atomic theories.)
The theory of natural selection explained how it
was possible for undirected (‘ random’) mutations
to lead to apparently purposive or goal-directed
changes in biological species. The theory of genes
explained how it was possible for offspring to
inherit some but not all of the characteristics
of each parent, and for different siblings to inherit different combinations. The theory that
atoms were composed of protons, neutrons and
electrons explained many of the possibilities
summarised in the periodic table of the elements,
and how it was possible for one element to be
transformed into another. Einstein’s theories of
~elativity explained how it i~ possible for mass
and energy to be interconvertible, and for light
rays to be curved even in a vacuum. Other
bilities explained before specimens were produced
include lasers and superconductivity.

The examples given so far are theories which not
only explained pOssibilities, but also contained
enough detail to make prediction and in some cases
control possible. In the case of the human sciences
this is rare. Marx’ s social theories explained
how. it was possible for large numbers of people to
collaborate peacefully in social and economic practices against their own interest. He also explained
how it was possible for such systems to generate
forces tending to their own overthrow. Popper has
tried to explain how it is possible for the growth
of scientific knowledge to be based on rational
comparisons and assessment of theories, even though
no theory can ever be proved to be right or even
probable. Chomsky’s theory that human minds contain representations of generative grammars explains how it is possible for sentences never before heard or uttered nevertheless to be part of a
person’s language. The t~o~ (see T. Winograd,
1973) that human minds contain certain sorts of
procedures or programs explains how it is possible
for new sentences to be produced or understood.

Freud’s theories explained how it is possible for
apparently meaningless slips and aberrations of
behaviour to be significant actions. Piaget’s
th~pries about the structure of many familiar concepts explain how it is possible for a child to show
in some behaviour that he has grasped the concept
and in others that he has not.

Known possibilities for which explanations are
still lacking include the rollowing. The possibili ty of the growth of an oak from an acorn or a
chicken from an egg. Fragments of the mechanisms
are of course understood already, but there is as
yet no explanation of how such an apparently simple
structure as a seed or fertilised ovum can control
its own development in such a way as to produce
such an apparently complex structure as a plant or
animal. Another unexplained possibility is the
evolution of animals with specific intelligent
abilities (like the ability to learn to use tools,
or to learn to use language) from species lacking
these abilities, and in particular the evolution
of human beings. In the case of human psychology,
there are very many possibilities taken for granted
as part of common sense, yet still without even
fragmentary explanations, for instance the poSSibility of a newborn infant learning whateyer human
language happens to be spoken around it, the
possibility of relating one’s actions to tastes,
preferences, principles, hopes, fears~ knowledge,
abilities, and social commitments, and the possibility of changing one’s moral attitudes through
personal experience.


Formal requirements for explanations of possibilities
The explanations listed earlier may not be correct
explanations, but they at least meet formal conditions for explaining certain possibilities, or
perhaps would do if precisely formulated. These
conditions will be described below. They are generalisations and elaborations of the basic idea,
familiar from writings of philosophers like popper,
Hempel and Nagel, that to explain something by
means of a theory is to deduce it from the theory,
perhaps with some additional premisses. As normally formulated, this assumes that both the theory
and what it explains are expressed in the form of
sentences, using natural language supplemented by
the technical language of the science concerned.

It is also normally assumed that the deduction is
logical, that is the inference from theory to what
it explains can be shown to be valid according to
the rules of inference codified by logicians.

(This is sometimes generalised to permit cases
where the inference is only probabilistic.)
This concept of deduction and the related notion
of explanation needs to be generalised in two ways.

First of all, other means of representation besides
sentences may be used, such as maps, diagrams,
three-dimensional models or computer programs.

Sec~ndly, the forms of inference include not only
the logical forms (like ‘All A’s are B’s, All B’s
are C’s. Therefore All A’s are C’s’), but also the
manipulation of other representations (see Sloman,
1971a). An example is the manipulation of diagrams
representing molecular structures, in order to explain the possibility of chemical reactions, like
the production of water from hydrogen and oxygen.

On this view the use of models and so-called
‘analogies’ in science is simply a change of language: one configuration is used to represent another.

All the usual talk about isomorphism of models in
this context is as misc;:onceived as the theory that
sentences in natural language must be isomorphic
with things they describe: there are many more
kinds of non-verbal representations than isomorphic models’. (See Goodman, 1968; Clowes, 1971; and
Toulmin, 1953. I was helped to see all this by an
unpublished paper by Max Clowes, called ‘P.aradigms
and syntactic models’.)

Further requirements for explanations of
We now have a minimal requirement for a theory T
formulated in sentences or other symbolic apparatus
to be an explanation of some range of possibilities,

(1) Statements or other representations of the
range of possibilities should be validly derivable
from T, according’to whatever criteria ‘far validity
are associated with the ‘language’ of T.

Perhaps one of the most important illustrations
of this is the use of the theory of bonds between
atoms (the theory of valencies) to explain the pos~sibility of a very large number of chemical compounds and transformations. Knowing the kinds of
bonds into which the various atoms can enter, one
can generate representations of large numbers of
chemical compounds, and chemical reactions, using
diagrams of the sorts mentioned previously. Here
one range of (relatively primitive) possibilities
is used to explain another range. The possibility
of many kinds of ‘ atoms with different chemical
bonding potentials was itself explained later on
by a more economical theory whic~ assumed atoms
could be made up of a nucleus coataining positively
charged protons, neutrons with n.p charge, and

electrons with negative charge. Thus, postulating
a small number of primitive subatomic components
and principles according to which they could be
combined into atoms, physicists could generate
representations of a wide range of possible atoms,
and therefore of possible molecules antl reactions.

These theories eventually had to be revised and
refined of course, but that does not affect the
point that at least part of the scientific function
of those theories ~le they survived was to explain a range of possibilities according to criterion (1). While they worked, they provided ‘generative grammars’ for known ranges of possibilities.

However, there are additional requirements if T
is to be a good explanation of the possibilities in
question. Rival theories are assessed according
to how well they meet these additional requirements,

(2) The theory should be as definite as possible:

i.e. there should be a clear demarcation between
what it does and what it does not explain.

(3) T should be general, i.e. it should eXPlain
‘many different possibilities, preferably including
some possibilities not known about before the theory was invented. This criterion should be used
with caution. Insofar as a theory generates some
possibilities not yet established by actual instances, efforts should be made to find or create
instances, and they should not be types of things
thought to be impossible because of some more
general theory. If repeated efforts to find actual
instances fail, this does not disprove the theory,
but it does reduce its credit. So a theory should
not explain too many things.

(4) T should account for fine structure: i.e. the
descriptions or representations of possibilities
generated by T should be rich and detailed.

(5) T should be non-circular: i.e. the possibilities assumed in T should not be of essentially the
same character as the possibilities T purports to

(6) The derivations from T should be rigorous:

i.e. within the range of possibilities explained by
T, the procedures by which those possibilities are
deduced or derived should be explicitly specified
so that they can be publicly assessed, and not left
to the intuitions of individuals •.

(7) The theory T should be plausible: that is, insofar as it make? any assertions or has any presuppositions about what is the case or what is
possible, these should not contradict any known
facts. However, sometimes the development of a
new theory may lead to the refutation of previously
widely held beliefs, so this criterion has to be
used with great discretion.

(8) The theory should be economical: i.e. it should
not include assumptions or,concepts which are not
required to explain the possibilities it is used
to explain. Sometimes economy is “taken to mean the
use of relatively few, concepts or assumptions, from
which others can be d~rived as necessary. The
latter is not always a good thing to stress, since
great economy in primitive concepts can go along
with uneconomical derivations and great difficulty
Qf doing anything with the theory, that is, with
heuristic poverty.

(9) The theory should be rich in heuristic power:

i.e. the concepts, assumptions, symbo1isms, and
transformation procedures of the theory should be
such as to make the detection of gaps and errors,
the structuring of , problem-solving strategies,
the recognition of reIevant evidence, and so on,
easily manageable. (See McCarthy and Hayes, 1969

and my 1971a for more on this)

These criteria therefore indicate ways in which
theories explaining possibilities may be criticised rationally. For instance, one may be able
to show (by a logical or mathema~ica1 argument)
that the theory does not in fact generate the
range of possibilities it is said to explain.

(Nearly all psychological tlieories put forward to
explain known human possibilities, such as perception, fail on this point: the theories generate
the required range of possibilities only in the
mind of a sympathetic audience supplying a large
and unspecified set of additional assumptions.)
A theory may be criticised by showing that it explains too much, including things which .so far
appear to be impossible. The theory may not explain enough of the known fine structure of the
possibilities (like theories of speech understanding which don’t explain how hearers can cope wtth
complex syntactic ambiguities, or developmental
theories in biology which don’t explain how a
chicken’s egg can grow into something like its
mother or father in so many detailed ways). The
explanation may be circular, like theories which
attempt to explain human mental functioning by
assuming the existence of a sp~rit or soul with
essentially all the abilities it is intended to
explain. The theory may be so indefinite that it
is not clear what it does and what it does not

A theory may also be criticised less directly by
.c.riticising the specification of the range of
possibilities which it is meant to explain (e.g.

criticising the typOlogy on which it is based).

For instance the specification may describe a set
of structures in ways which are not related to
their functions, like describing sentences in terms
of transition probabilities between successive
words. Or the set of possibilities explained may
be shown to be only
sub range of some wider set of
possibilities which the theory cannot cope with.

For instance, a theory which explains how statements
are constructed and understood can be criticised if
it cannot be extended to account for questions,
commands, threats, requests, promises, bets, contracts, and other types of verbal communication
which are clearly functionally related to statements
in that they use related syntactic structures and
almost the same vocabulary. If it turns out that
a physical theory of the interactions of atoms
and their components can only explain the possibility of chemical reactions involving relatively
simple molecules, then that will show an inadequacy in the theory. Similarly, if an economic theory
can explain only the possibility of economic
processes occurring when there is a very restricted
amount of information flow in a community, then
that theory is not good enough. Finally, if a
theory of the function of moral language accounts
only for abusive and exhortative uses of that kind
of language, then it is clearly inadequate since
moral language can be used in a much wider range
of ways.

In some cases, whether a theory explaining some
specified range of possibilities satisfies these
criteria or not, or whether it satisfies them
better than a rival theory, is not an empirical
question. It is a question to be settled by logical and mathematical investigations of the struc~ure of the theory and of what can be derived from
it. Sometimes the theory is too complex for its
properties to be exhaustively surveyed. If so one
can only tryout various derivations or manipulations in test cases. This is partly analogous to
an empirical investigation in that the /’resu1 ts are :( ,i1~
always partial and cannOt be worked out in adv:e



by normal human reasoning.

Similar ly testing a
complex computer program may feel like conducting
some kind of experiment. Nevertheless, as already
remarked, the connections so discovered are not
empirical, but logical or mathematical in” nature.

(The criteria listed here can be justified by
showing how using them is necessary for furthering
the interpretative and practical aims of science
listed in part one.)

on the theory. A psychological theory may imply
that it is possible for a human being to count
backwards from 99 to 0 to the tune of ‘Silent night
holy night’, without being refuted merely by the
fact that nobody ever does this. Only a much more
complex theory, taking into account a rich set of
motives and beliefs, could ever be used to pred~ct
such a performance, and perhaps be refuted by its

Lack of predictive power, practical utility, or
Prediction and testing
refutability need not prevent the scientific
merits of an explanation of a range of possibiliA theory may meet the conditions above without
ties from being d~scussed rationally, and compared
being· of any use in predicting or explaining parti- with the merits of rival explanations, in accordcular events or in enabling events or processes to
ance with the criteria listed above. Nor does it
be controlled. This is why I have stressed the
prevent such a theory from giving deep insight, of
explanation of possibilities. Although it explains a kind which provides a firm basis for building
how certain sorts of phenomena are possible, the
more elaborate theories which do permit predictions
and explanations of particular events, and which
underlying mechanism or structure postulated may,
are empirically refutable.

at the time the theory is proposed, be unobservable, so that observation of its state cannot be
I therefore see no reason for calling such
theories nonsensical, as some of the logical posiused to predict actual occurrences of those phenotivists would, nor for banishing them from the
mena. Similarly, no techniques may be available
realm of science into metaphysics, as Popper does
for manipulating the mechanisms, so that the
(though he admits that metaphysical theories may
theory provides no basis for controlling the
be rationally discussable and may be a useful stimphenomena. For instance, the theory of evolution
ulus to the development of what he calls scientific
eXplains the possibility of a wide range of biotheories). I am.not here arguing over questions of
logical developments without providing a basis for
meaning: define ‘science’ as you will, my point
predicting them. Similarly, a theory explaining
remains that among the major merits of the generthe possibility of my uttering sentences of particular forms need not provide any~sis for predict- ally agreed most profound scientific theQries is
ing when I will utter anyone sentence, or for
the fact that they satisfy the above criteria for
making me utter it, or even for explaining exactly
being good explanations of possibilities, and
why I uttered the particular sentence I did utter
therefore give us good insights into the nature of
at a particular time. This is because the theory
the kinds of objects, events or processes that can
may simply postulate a certain kind of sentenceexist or occur in the universe~ If unrefutable
generating mechanism, available in my mind as a
theories are to be dubbed ‘metaphysical’, then what
resource to be used along with other resources.

I am saying is that even important scientific

theories have a metaphysical component, and that
How any particular resource is used on any particular occasion, may be the result of myriad compthe precision, ge~eral~ty, fine structure, nonlex interactions between such factors as my purposes circularity, rigour, plausibility, economy an~
preferences, hopes, fea;s and moral principles, what heuristic power of the metaphysical component are
I believe to be the case at the time, what I know
among the objective criteria by which scientif~c
theories are in fact assessed (and should be
about the likely effects of various actions, how
assessed). The development of such ‘metaphysical’

much I am distracted and so on. The theory which
theories is so intimately bound up with the develexplains the possibility of generating and underopment of science that to insist on a demarcation
standing sentences need not specify all the interis to make a trivial semantic point, of no theoractions between the postulated mechanism and other
aspects of the m~d. So it need not provide a
etical interest. Moreover, it has bad effects on
the training of scientists.

basis for prediction and control.

This is true of any explanation of an ability,
Empirical support for ~lanations of possibilities
skill, talent; or power, in terms of a mechanism
Further, even though a theory explaining only cermaking it possible. The explanation need ,not
specify the rest of the aystem of which that retain possibilities is not refutable empirically,
source is a part, nor specify the conditions under
that does not mean that empirical evidence is
which the resource is used. And even if it does,
wholly irrelevant to it. For instance, if a kind
the specification need not refer to either observof possibility explained by the theory. is observed
able conditions or manipulable conditions. So such for the first time after the theory was constructed,
explanations of possibilities, though they contrithen this is empirical corroboration for the
bute to scientific understanding, need not contri~
theory, even though the theory did not specify
bute to predictions of actual events.

that the phenomenon ever would occur ,or that it
It is not possible to refute such a theory, if it would occur in the particular conditions in which
merely explains possibilities, and entails or exit did. Observing an actual instance of a possiplains no impossibilities. For it is a fact about
bility explained by some theory provides support
the logic of possibility that ‘X is possible’ does
for that theory at least to the extent of showing
not entail ‘X will occur at some time or other’.

that there is something for it to explain: it shows
that the theory performs a scientific function.

Similarly ‘X never occurs’ does not entail ‘X is
impossible’. Newtonian mechanics entails that it
However, the support adds to previous knowledge
only if it is a new k,ind of possibility. Mere
is possible for same very large body passing near
repetition of observations or experiments does not
the earth to deflect the earth from its orbit, and
it explains this possibility: but the fact that this increase support for a theory: it merely checks·
never occurs casts no doubt on the theory. Similar- that no errors.were made in previous instances.

In those contexts all the normal stress on
ly,a grammatical theory may explain the possibirepeatability of scientific experiments is unlity of the utterance of a certain rather complex
necessary and has misled many psychologists and
English sentence, and even though nobody ever
utters that sentence naturally, this casts no doubt social scientists into making impossible demands


of empirical studies of man and society. Repetition may be a useful check on whether the phenomenon really is possible (since it permits more
independent witnesses to observe it), and it provides opportunities for more detailed examination
of exactly what occurred, but is not ~ogically
necessary. If a phenomenon occurs only once, then
it is possible, and its possibility needs explaining. Any explanation of that possibility is ther~e-.~
fore not gratuitous, and the only question that
should then arise is not whether the explanation
is science or pseudo-science, or metaphysics~ but
whether a better explanation can be found for the
same possibility, that is, an explanation meeting
more of the criteria (2) to (9) above; or perhaps
serving additional scientific aims besides explaining’possibilities.

The frantic pursuit of repeatability and statistically significant correlations is based on a
belief t~t science is a search for laws. This
has blinded many scientists to the need for careful
description and analysis of what can occur, and
for the explanation of its possibility.

Instead they try to find what always occurs – a
much harder task – and usually fail. Even if something is actually done by very few persons, or only
b¥ one, that still shows that it is possible for a
human being, and this possibility needs explana.tion as much as any other established fact. This
justifies elaborate and detailed investigation and
analysis of particular cases: a task usually shirked
because of the search for statistically significant
correlations. Social scientists have much to learn
from historians and students of literature – despite
all the faults of tile latter.

I have gone on at such great length about describing and ~plaining possibilities because the
matter is not generally discussed in bocks on
philosophy of science. But I do not wish to deny
the importance of trying to construct theories
which can be used to explain and predict what
actually OCCUl,-s, or which explain impossibilities
and observed regularities. Of toJO t..l-}eories explaining the same range of possibilities, one which
also explains more impossibilities and permits a
wider variety of predictions and explanations of
actual events to be made on the basis of observation, is to be preferred since it serves to a
greater degree ~~e aims of science listed previous1y. I think it is quite premature to s~ek such
predictive explanations in psychology and social
science: these sciences still seem too far from
having good explicit descriptions and explanations
of possibilities we all know about, as linguistics
was until recently.

This discussion is still very sketchy and unsatisfactory. Much finer. description and clas$ification of ~ifferent sorts of explanation is
required. But enough for now!


them. I hope -I have said enough to refute these

claims. Roughly, our disagreement seems to. hinge
on Popper’s view that the only place for rationaiity in science is in the selection from among
hypotheses expressible in a given language, whereas
I have tried to show that there are rational ways
of deciding how to extend a language, and therefore
how to extend the set of expressible hypotheses.

I admit that there are still serious gaps in my
discussion: a theory of concept-formation is still

Finally, even if it is agreed that science uses
rational means to pursue the aims described here,
the question arises: are these aims rational? Is
it rational to pursue them? I believe there is no
answer to this. If someone genuinely prefers the
life of a mystic or hermit or ‘primitive’ tribes~
man to the pursuit of knowledge and understanding
of the universe, then that preference must be
respected. However, I believe that the aims and
criteria described here are pa]:’t of th~ mental
mechanism with which every ·human child is born but for which it would not be p~ssible to learn
all that human children do learn. So one can
reject science only after one has used it, however
unconsciously, for some years.

Chomsky, N. (1957) Syntactic structures, Mouton,
‘The Hague
Clowes, M. (1971) ‘On Seeing Things’, Artificial
Intelligence, 2, 1971
Gibbs, B,.R. (1970) ‘Real Possibility’, American
Philosophical Quarterly, October 1970
Goodman, N. (1968) Languages of Art, Oxford
University Press
Kant, I. (1781) Critique of Pure Reason, translated
by N.K. Smith, Macmillan, 1929
Kuhn, T.S. (1962) The Structure of Scientific
Revolutions, Phoenix Books
McCarthy, J. and Hayes, P. (1969) ‘Some philosophical problems from the standpoint of artificial
intelligence’, in Machine Intelligence 4, ed. D.

Michie and B. Meltzer, Edinburgh University Press
Popper, K.R. (1972) Objective Knowledge, Oxford
University Press
Sloman, A. (197la). ‘Interactions between philosophy
and Artificial Intelligence: the role of intuition
and non-logical reasoning in intelligence’, in
Proc. 2nd International Joint Conference on Artificial Intelligence (The British Computer Society,
1971), and in ~~tificial Intelligence 2 pp209-25, 1971
Sloman, A. (197lb) ‘Tarski,Frege and the Liar Parado~’, in Philosophy XLVI, April 1971
Toulmin, S. (1953) The Philosophy of Science,
Watson, J. (1968) The Double Helix, Penguin Books
Winograd, T. (1973) Understanding Natural Language
Edinburgh University Press
Wittgenstein, L. (1956) Remarks on the Foundations
of Ma thematics, Basil Blackw,all # Oxford

For reasons which I do not fully understand,
Popper is apparently strongly opposed to all this
talk of concepts and possibilities (see, for instance, pp123-4 of his (1972) where he describes it
as an error to think that concepts and conceptual
systems or problems about meaning are comparable in
importance to theories and theoretical systems, or
to problems 9£ truth.) As far as I can tell, his
argument rests on the curious assumption that concepts or meanings are purely subjective things, and
that the only objective criteria by which they can
be assessed or criticised are ones which concern·
the truth of ,statements and theories containing


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