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The Logic of Scientific Discovery 163
3. NATURAL NECESSITY AND NATURAL KINDS: THE
STRATIFICATION OF NATURE AND THE STRATIFICATION OF
SCIENCE
In the process of the establishment of a law of nature
three questions may be asked:
(i) is there an empirical regularity which
constitutes a prima facie candidate for a law ?
(ii) is there some reason, other than the
regularity, why the
164 A Realist Theory of Science
predicates instantiated in the law-like statement should
be conjoined ?
(iii) is this reason located in the enduring powers
of things and the transfactually active mechanisms of
nature?
If the answer to (i) is yes we have what might
be called a `protolaw'.38 If the answer to (ii) is yes we
have strong grounds for a law. If the answer to (iii) is
yes we have a law. Typically of course the reason in
question in (ii) will be provided by a model of the
connection between antecedent and consequent, putative
cause and putative effect. The transition from (ii) to
(iii) typically occurs when a realist interpretation of
the mechanism posited in the model becomes acceptable.
The answers to (i) - (iii) correspond of course to
three levels of criteria for law, viz. those specified by
the classical empiricist, transcendental idealist and
transcendental realist philosophies of science. At the
Humean level laws just are empirical regularities. At
the Kantian level both (i) and (ii) must be satisfied.
Here we have what might be called the dual criterion
theory of law.39 I have already noted its vulnerability
to Humean counterattack. At the level of transcendental
realism, a distinction is drawn between the empirical
identifiability and the universal (transfactual)
applicability of laws; and the latter is seen to be a
condition of the possibility of the former. As the
application of laws in open systems is justified, and
presupposed by the intelligibility of experimental
activity, the existence of an empirical regularity or a
constant conjunction of events is now not even necessary
for the ascription of a law (see Table 3.1 below). I have
argued that it is only at this level that a distinction
Table 3.1
Status of Constant Conjunction of Events
Necessary Sufficient for Law
classical empiricism Y Y
transcendental idealism Y N
transcendental realism N N
38 R Harre, op. cit., p. 132.
39 This theory is most clearly stated in R. Harre,
op. Cit., Chap. 4. Although Harre is I think
logically committed to, and may he prepared to accept,
transcendental realism in the form in which it is
developed here, he does not say how laws `explain away
counter-instances' and so achieve universality,
ibid. p. 92.
The Logic of Scientific Discovery 165
between necessary and accidental sequences can be
sustained. A sequence E_a.E_b is necessary if there is a
generative mechanism M such that whenever E_a, E_b tends
to be produced; a sequence is accidental if this is not
the case. Their difference is represented in Diagram 3.4
below. Most events occur in open systems and must be
conceived, as argued in 2.6 above, as `conjunctures'.
Diagram 3.4 (not typed in)
This is illustrated in Diagram 3.5 below. Necessity as
such, like universality, is thus ascribed essentially to
the activity of the mechanism; and only derivatively to
some particular event sequence. For the result of the
activity of the mechanism will
Diagram 3.5 (not typed in)
166 A Realist Theory of Science
in general be co-determined by the activity of other
mechanism too.40
Now these three levels of criteria generate and are
generated by different views of science. Thus whereas
the classical empiricist will ask merely:
(i)* is there a regularity such that whenever C then
E?
The transcendental idealist will ask in addition:
(ii)* given a regularity, is there an explanation
such that we can render it intelligible to ourselves that
whenever C then E?.
The transcendental realist will however, after making
an essential correction, go one step further and ask:
(iii) out of the plausible explanations for this
regularity, is there one which correctly describes the
mechanism by means of which, upon the occurrence or
obtaining of C, E tends to be produced? That is to say,
the transcendental realist will demand that models be
tested not just for plausibility but for truth; i.e. for
their adequacy in correctly describing the real
generative mechanism at work (if the connection between C
and E is necessary) such that when C occurs, E tends to
be produced (is produced in the absence of interfering
causes or the transformation of M). That real things and
generative mechanisms must exist can be established by
philosophical argument. It is the job of the scientist
to discover which ones actually do. Given the
identification of some prima facie non-random pattern in
nature or protolaw the scientist thus builds up ideas of
various plausible hypothetical mechanisms by the creative
employment of his imagination (cf. Diagram 3.6) and
subjects these ideas to rigorous theoretical criticism
and empirical test. These three phases of science are of
course those represented in Diagram 3.1 on page 114
above.
For the transcendental realist then a model has a
relationship with its subject as well as with its source.
And it is within the
40 It should be remembered that transcendental realism
not only warrants subjunctive and counterfactual
statements (where antecedents are uninstantiated) but
normic and transfactual ones (where consequents may be
unrealised). This is another nail in the coffin of
deductivism. For at level (iii) a law may be upheld even
when P is true and Q is false; which is of course the
only case, according to the principle of material
implication, when a conditional is false. The moral is
that falsification always depends upon the non-formal
requirement that the system in which the putative
counter-instance occurs be closed.
The Logic of Scientific Discovery 167
nexus formed by this double articulation that new
knowledge is produced. For new knowledge is doubly
articulated, articulated in two dimensions (transitive
and intransitive): it is a socially produced knowledge of
a natural (man-independent) thing. It is this bipolarity
that a model expresses in standing in two sorts of
relationship: a relationship of *analogy* with its source;
and a relationship of *adequacy* (when it is) with its
subject matter.
Diagram 3.6 (not typed in)
Many philosophical problems arise from a misunderstanding
of the second relationship. It is not a relationship of
correspondence; the terms of the relationship are not
necessarily like each other, though pictures and iconic
models may play an important role in scientific
thought.41 Moreover there are no general philosophical
criteria for such judgements of adequacy; they are
necessarily intrinsic to the particular science
concerned. Analogy is one of the possible relationships
that models may have with respect to their source. The
existence of the first type of relationship (in the
transitive dimension) is important in establishing both a
constraint on the number of possible explanations42 and
an indispensable means of their production.
41 See e.g. N. R Hanson, `A Picture Theory of Theory
Meaning', The Nature and Function of Scientific Theories,
ed. R. G. Colodny, pp. 233-73.
42 I have argued in paragraph 2 above that without such a
constraint on the content of possible explanations,
sorting them with respect to their plausibility, there
will be an infinite number of possible explanations, even
of equal simplicity. The plausibility of a possible
explanation cannot be identified by purely syntactical or
formal criteria alone but depends upon a complex
relationship between what is so far known about the
process generating the behaviour in question and
established explanation patterns drawn from
analogous fields. It is thus in part a function of the
existing knowledge in which the predicates occurring in
the possible explanations are already embedded, so that
the paradoxes of confirmation, etc. that flow from the
insertion of artificial predicates into
already-functioning and well-connected scientific
contexts cannot (at least at that point of application)
arise. Cf. R. Harre, `Surrogates for Necessity', Mind
1973, pp. 355-80.
168 A Realist Theory of Science
It cannot be described at all adequately as one of
coherence; and here again no general philosophical
criteria can be laid down for it. Science is work that
requires creative intelligence, and there can be no
mechanical surrogate for that. The idea of an automatic
science is a will-o'-the-wisp that the philosophy of
science has pursued, with damaging consequences, since
Bacon's search for a `sure and certain method' that would
eliminate the need for human thought, which of course
inevitably entails the possibility of human error.
Most science proceeds by way of a two-tiered method
designed to identify invariances in nature, normally
under conditions which are experimentally produced and
controlled, and to explain them by reference to enduring
mechanisms.43 It is in the movement from the
identification of an invariance to the mechanisms and
structures that account for it that the logic of
scientific discovery must be found. Thus the observable
reactions of chemistry, which are represented in
textbooks by formula such as 2Na + 2HCl = 2NaCl + H_2,
are explained by reference to the atomic hypothesis and
the theory of valency and chemical bonding. The patterns
which constitute the explananda of the theory of valency
are needless to say by no means superficially obvious or
readily available. Both the concepts and the substances
and conditions had and have to be worked for, produced in
the social activity of science. The theory itself sets
out to describe the causal mechanisms responsible for the
overt behaviour of the substances. Once its reality has
been established (which justifies our assuming that
chemical bonding occurs and the laws of chemistry hold
outside the laboratory) and the consequences of the
theory have been fully explored, the next task consists
in the discovery of the mechanisms responsible for
chemical bonding and valency. This has been explained in
terms of the electronic theory of atomic structure. Once
the reality of this explanation has been established,
science moves on to the discovery of the mechanisms
responsible for what happens
43 Ibid. p. 366.
The Logic of Scientific Discovery 169
in the sub-atomic microcosm of electrons, protons, and
neutrons; and we now have various theories of sub-atomic
structure. The historical development of chemistry may
thus be represented by the following schema:
Stratum I 2Na + 2HCl = 2NaCl + H_2
explained by
Stratum II theory of atomic number Mechanism 1
and valency
explained by
Stratum III theory of electrons and Mechanism 2
atomic structure
explained by
Stratum IV [competing theories of sub- [Mechanism 3]
atomic structure]
It should be noted that the historical order of the
development of our knowledge of strata is opposite to the
causal order of their dependence in being. No end to
this process of the successive discovery and description
of new and ever deeper, and explanatorily more basic,
strata can be envisaged. Other sciences reveal a similar
open-ended stratification. Geometrical optics is
explained in terms of Young and Fresnel's wave optics;
which is explained in terms of the electromagnetic theory
of light; which can be explained in terms of the quantum
theory of radiation.44
A general pattern of scientific activity emerges
from this. When a stratum of reality has been adequately
described the next step consists in the discovery of the
mechanisms responsible for behaviour at that level. The
key move in this involves the postulation of hypothetical
entities and mechanisms, whose reality can then be
ascertained. Such entities need not be smaller in
size,45 though in physics and chemistry this has normally
proved to be the case. The species of explanation here
identified itself falls under a wider genus: in which the
behaviour of individuals is explained by reference to
their natures and the
44 Cf. M. Bunge, op. cit., p. 38.
45 Cf. G. Schlesinger, `The Prejudice of
Micro-Reduction', B.J.P.S., Vol. 12, pp. 215-24.
170 A Realist Theory of Science
conditions under which they act and are acted upon (see
2.3 above).
Now for the transcendental realist the
stratification this form of explanation imposes upon our
knowledge reflects a real stratification in the world.
Without the concept of real strata apart from our
knowledge of strata we could not make sense of what the
scientist, striving to move from knowledge of one stratum
to knowledge of the next, is trying to do: viz. to
discover the reasons why the individuals which he has
identified (at a particular level of reality) and whose
behaviour he has described tend to behave the way they
do. Without this concept the stratification of science
must appear as a kind of historical accident, lacking any
internal rationale in the practice of science (if indeed
it is not denied altogether in a reductionist and
ultimately phenomenalist account of science).
As it is clear that the hypothetical entities and
generative mechanisms imagined for the purposes of
theory-construction must initially derive at least part
of their meaning from some other source (if they are to
be capable of functioning as possible explanations at
all) theories *must* be already understood before
correspondence rules for them are laid down.46 Equally
this means that the descriptive terms must have initially
possessed a meaning independent of them. This enables us
to see how meaning-change is possible, and indeed if the
independence of predicates is denied, inevitable in the
transitive process of science. Similarly we can see how
knowledge of newly discovered strata may correct
knowledge of less fundamental strata, as concepts and
measurement techniques are refined. Now if changing
knowledge of strata is to be possible the strata must not
change with our knowledge of them. Thus the concept of
real strata apart from our knowledge of them is necessary
if both the ideas of scientific structure and scientific
change, which are central to recent critical philosophy
of science, are to be intelligibly sustained. More
generally, acknowledgement of the real stratification of
the world allows us to reconcile scientific discovery (of
new strata) with scientific change (of knowledge of
strata).
Now the stratification of the world must be assumed
by the
46 Cf. K. Schaffner, `Correspondence Rules', Philosophy
of Science, Vol. 36 (1969), pp. 280-90.
The Logic of Scientific Discovery 171
scientists working in any field, to be in principle
unbounded. For it will always be possible for him that
there are reasons, located at a deeper level, for the
phenomena he has hitherto identified and described. But
his knowledge may be in practice bounded by
semi-permanent technical or conceptual problems or by the
domain assumptions of his particular science; or by the
fact that reality is itself bounded at the level
knowledge of which he has attained. However, if the
stratification of the world has an end, i.e. if there are
`entities' which are truly ultimate - and I can see no
reason for supposing this must be so - and the scientist
has achieved knowledge at that level, he can never know
that the level is ultimate. For it will still remain
possible for him that there are reasons, located at a
still deeper level, for the causes of the phenomena he
has succeeded in identifying and describing. I will
return to this point below.
Now the only kind of necessity that holds between
events is connection by a generative mechanism. But
there are two other concepts of necessity applicable to
the objective world order: there is the necessity
implicit in the concept of a law, i.e. in the activity of
a generative mechanism as such or the exercise of a
thing's tendencies irrespective of their realization; and
the necessity implicit in the concept of a thing's real
essence, i.e. those properties or powers, which are most
basic in an explanatory sense, without which it would not
be the kind of thing it is, i.e which constitute its
identity or fix it in its kind. The first concept of
`natural necessity' is clearly derivative from the
second, dependent upon the contingent feature of the
system in which the thing's behaviour occurs, viz. that
it be closed (see 2.4 above). I am therefore going to
refer to the second as the concept of natural necessity,
and the third as the concept of natural kinds. Knowledge
of natural necessity is expressed in statements of causal
laws; knowledge of natural kinds in real definitions.
But natural kinds exist and naturally necessary behaviour
occurs independently of our definitions and statements of
causal laws.
Now in the transition from knowledge of any one
stratum to knowledge of the next, knowledge of three
levels of the objective world order is progressively
obtained: of relations between events, of causal laws and
of natural kinds. I am going to refer to these three
levels as the Humean, Lockean and Leibnizian
172 A Realist Theory of Science
levels respectively. The transcendental idealist, as
well as the classical empiricist, is, in virtue of his
ontological commitment, restricted to the first level of
knowledge of the objective world order. I shall argue
that even at the Leibnizian level science remains
empirical, so that the transcendental argument of Chapter
1 remains valid; and that even at that level the
deductively justified prediction of events is impossible,
so that the critique of philosophy of science contained
in Chapter 2 continues to apply with undiminished force.
Moreover I shall argue that the concepts, such as that of
natural powers, that we need to render intelligible the
transition to the Leibnizian level remain categorically
valid even at that level.
At the *Humean* level a pattern is identified or an
invariance is produced. (This, we know, empirical
realism cannot sustain.) We thus have a protolaw (at let
us say Stratum I). This is to be explained by reference
to the circumstances and nature of the thing whose
behaviour is described. The scientist never doubts for a
moment that something is generating the effect in
question. His problem is: what is? That is, why does x
behave the way it does, viz. B, in conditions C_1...C_n?
The first step in the scientific explanation of B is
to ascribe a power (or liability) of x to B, i.e. to do
(or suffer) phi. This is to say, very roughly, that x
does phi in virtue of its nature N.47 The next step is
thus to investigate N (defining Stratum II). This
involves inter alia creative model-building and rigorous
empirical-testing (cf. Diagram 3.1). As a result of this
investigation we may say x comes to do phi in virtue of
its having a certain constitution or intrinsic structure,
e.g. genetic constitution atomic structure or electric
charge. Now it is contingent that x has the nature
(e.g. constitution or structure) that it has. But given
that it has, It is necessary that it behaves the way it
does. One criterion of this is our capacity to deduce
the tendency to
47 Given the B is law-like and allowing for open systems
we must say: x tends to do phi in virtue of its nature N.
A discussion of the rather complex relationship between
tendencies and powers must be postponed to the appendix
to this chapter. For the moment they may be regarded as
a class of powers whose exercise is normically qualified.
But this is not a complete analysis. For a power may be
exercised when the behaviour is not law-like, so that it
would be wrong to attribute a tendency. The logic of
power ascriptions, their role in science and the
ontological status of powers will be discussed below.
The Logic of Scientific Discovery 173
B from N. This is the *Lockean* level of knowledge. Note
that at this level it is still contingent that the thing
has the structure that it has.
Now at the third *Leibnizian* level possession of that
structure or constitution comes to be regarded as
defining the kind of thing that x is. Now it is
necessary that x has the structure it has if it is to be
the kind of thing it is. It is no longer contingent that
hydrogen is a gas with a particular atomic structure;
rather anything possessing that structure is hydrogen.
That is, the criterion for the application of the concept
`hydrogen' ceases to be the lightest gas and become
instead possession of that structure. At this level the
only contingent questions are whether and where things of
a given kind exist.48 But note contingency still lies in
the flux of the circumstances in which things act, so
that events are still not deductively predictable. That
is, the `contingency' of events deriving from open
systems applies even at the Leibnizian level, so that
laws must still be formulated as tendencies (whatever
their scope of application). Moreover it is important to
see that knowledge at the Leibnizian level is, or may be,
attained empirically. We may discover, quite
empirically, that the most important explanatory property
or real essence of hydrogen, identified as the lightest
gas, is its atomic structure; and then attempt to express
this discovery in a real definition of hydrogen. Once
more the importance of viewing science as a process in
motion is clear. For if we stay at any one level, phase
or moment of science the idea that a definition may be
arrived at empirically will appear absurd. If it is
accepted, however, the reason why the laws of nature
cannot be deduced a priori from self-evident axioms
becomes clear. For the axiomatic base of a science at any
moment of time, at any stratum of reality, is something
that has had to be worked for, produced, as part of the
irreducibly empirical process of science.
Scientists attempt to discover what kinds of things
there are, as well as how the things there are behave; to
capture the real
48 The second question is both distinct from the first
and important. Because it raises the question of the
range or scope of application of the statements
expressing the tendencies of the individuals
concerned. It cannot be assumed that all tendencies will
be spatio-temporally universal; for individuals and kinds
may be transformed in time and bounded in space. A law
may of course be universal (transfactually applicable)
within its range and restricted in this way.
174 A Realist Theory of Science
essences of things in real definitions and to describe
the ways they act in statements of causal laws. The real
essences of things are their intrinsic structures, atomic
constitutions and so on which constitute the real basis
of their natural tendencies and causal powers. Thus
there is no conflict between explanatory and taxonomic
knowledge. Rather, at the limit, they meet in the notion
of the real essences of the natural kinds, whose
tendencies are described in statements of causal laws.
At the Leibnizian level statements of law are
substitution instances of necessary truths about the
individuals to which the refer. For any individual which
did not behave in that way would not be an individual of
that kind. They may thus be regarded as analytic truths.
But they are arrived at in the transitive process of
science a posteriori, by empirical means. Thus a fully
dynamic philosophy of science must take seriously the
question `how is analytic knowledge arrived at a
posteriori possible ?' To this question I will return in
paragraph 5 below
The situation at the Humean level is rather like
that faced by the citizens of Koenigsberg who knew, from
experience, that there was no way of crossing each of the
town's seven bridges just once.49 See Figure 3.1 below.
At the Lockean level this fact is deducible from the
topology of Koenigsberg, given Euler's theorem. At the
Leibnizian level, there is a necessary truth about a
certain physical set-up, whether or not there is a town
called `Koenigsberg' or any town at all to which it
applies. At the Leibnizian level, Mendeleyeev was able
to deduce from his Periodic Table, interpreted as dealing
with atomic number and valency, the properties of several
new elements. But it remained contingent whether, and if
so where, there were elements in the world to which his
predictions applied. Certain chess games have
Figure 3.1. The Seven Bridges of Koenigsberg
(not typed in)
49 Cf. M. Hollis and E. J. Nell, Rational Economic Man,
Chap. 7.
The Logic of Scientific Discovery 175
only one possible solution. But it remains contingent
whether they are ever played.
The concept of powers has played a key role in our
analysis of science's transition from knowledge of one
stratum to knowledge of the next. To ascribe a power is
to say that a thing will do (or suffer) something, under
the appropriate conditions, in virtue of its nature. This
is not, as is so often claimed, a pseudo-explanation50 or
a purely verbal formula.51 Rather it is an indication of
work to be done. Moliere's doctors in Le Malade
Imaginaire have often been ridiculed for speaking of
opium as possessing a `dormitive virtue'. But in doing
so they left open the possibility of an investigation, at
some future date, into the nature of opium without
committing themselves to what would doubtless have been,
for them, a rash conjecture at the time. Moreover it is
far preferable to the Humean alternative, viz. that
whenever men smoke opium they fall asleep. For in the
first place, the latter is untrue. Secondly, it is less
informative. It might be a complete accident that
everyone in the den is asleep: the powers formula rules
this out. It says that there is something about opium in
virtue of which when men smoke it they tend to fall
asleep. The connection is necessary. But it is only a
tendency. Thirdly, the Humean formula is regulatively
useless. The powers statement is by contrast quite
suggestive. For it indicates the need for an
investigation into the chemical properties of opium and
the way they induce sleep in men.52 In context, it
constitutes an open admission of ignorance. The Humean,
on the other hand, must pretend that once he has his
generalization there is nothing more to be known. And if
he should stumble upon a higher-order generalization this
can be, for him, only accidentally related to the
original one: there is no inner logic connecting the two,
or rationale by which science moves from the first to the
second.
To this it might be objected that the concept of
powers does not figure in the discourse of science. This
is true. And the reason for it is of course that the
scientist, unlike Moliere's doctors, is never just
content to ascribe a power but moves
50 See e.g. A. Flew, An Introduction to Western
Philosophy, p. 49; or E. Nagel, op. cit., p. 37.
51 See e.g. L. Kolakowski, Positivist Philosophy, p. 34.
52 Cf. R. Harre, Principles of Scientific Thinking,
pp. 274-5.
176 A Realist Theory of Science
immediately to the construction of possible explanations
for it with the paradigms and other instruments of
thought at his disposal. That is his job. (Sometimes,
however, when we are completely at a loss we do just
ascribe a power.) The concept of powers is introduced
precisely to describe this normally instantaneous (or
simultaneous) and unselfconscious response of the
scientist to the identification of protolaws; it
represents, if you like, an attempt to reconstruct the
internal rationality of the inter-strata move. The
concept of powers is not intended to figure in the
discourse of science, but in the discourse of the
philosophy of science (which is the former's rational
reconstruction).
It should perhaps be stressed here that the stages
of my rational reconstruction of the process of
scientific discovery represent phases of scientific
activity; they cannot be identified with moments of
chronological time. Thus most scientific work must
occur, for reasons I will bring out in paragraph 4, in the
context of a research programme designed to show that on
the supposition of the mechanism M the field of phenomena
can be rendered intelligible. Thus the identification of
a protolaw normally depends upon the prior existence of a
conjecture or a hypothesis of a mechanism intended to
function as a possible explanation for the presumed
protolaw.53
To ascribe a power is to say that there is something
about the thing, which may be unknown, in virtue of which
it behaves the way it does. The grounds for the
ascription of a power must thus be stronger than the mere
occurrence of a regularity. For we must possess some
reason to suppose the connection necessary (though in the
limiting case this may just be the invariance of an
experimentally produced result). It is because it
indicates the power-ascriber's belief in the existence of
a reason, located at the next highest level of inquiry
(in the nature of the thing), whether
53 Cf. K. R. Popper, Conjectures and Refutations,
Chap. 5. However the protolaw itself when it finally
emerges, pari passu with its explanation after the
limitations and modifications necessitated by the
experimental process, may be in a form far more complex
and refined than that in which it was originally
conceived (cf. S. Koerner, Experience and Theory,
passim). The normal response to a (genuine)
counter-instance is modification within a continuous
research programme, rather than (as is implied by naive
falsificationism) the complete abandonment of the
original conjecture and its replacement by a totally
different one (cf. I. Lakatos, op. cit.).
The Logic of Scientific Discovery 177
or not the reason is currently known, that the concept of
powers, in pinpointing an essential moment in the
transition from knowledge of one stratum to knowledge of
the next, plays such a key developmental or
strata-bridging role. In this way, a powers statement is
a promissory note cashed in the development of science, a
schematic explanation filled out in the growth of our
knowledge.54
It is worth noting that the structure of a powers
ascription is well adjusted to accommodate both
falsification (obviously, as the hypothesized reasons may
be subjected to independent tests) and meaning-change
(less obviously). If meaning change is to be possible,
some elements of meaning must remain constant through the
change. Now if `x does B' is analysed as `x is of such a
nature N [defining Stratum II] that it will do phi in
conditions C_1 . . . C_n [defining Stratum I]' we may
allow that the meaning of `phi' remains constant while
the meaning of the N component changes between theories
(and vice-versa). This applies even in the case of
simple descriptive observational predicates such as
`blue'. For `x is blue' may be analysed as `x looks blue
[defining Stratum I] in virtue of its reflecting light of
a certain wavelength [Stratum II]'. The simple theory
that things look blue because they are blue may then be
replaced by the scientific theory that they tend to look
blue in normal circumstances because they reflect light
of wavelength 4400A. Subsequently we may allow the
latter to define the scientific use of `blue'; in which
case of course it is no longer contingent that blue
surfaces reflect light of that wavelength.
Now although the concept of powers serves this
essential developmental function, it cannot be reduced to
it. For when we have climbed up to Stratum II, we cannot
throw away the ladder, so to speak. To pursue the
analogy, the ladder is a rope, not a wooden one. For to
make a powers statement is to make a categorical
statement about the nature of the thing situated at the
level to which we have climbed. It is to make a
statement about possibilities which are possessed by the
thing quite categorically, whether they are known (or
actualized) or not. Dogs do not lose their power to bark
when we understand how they do so, just as glass does not
cease to be brittle when we know its molecular structure.
54 Cf. R Harre, op. cit., p. 275.
178 A Realist Theory of Science
The ontological bases of powers just are the
properties that account for them; i.e. the natures in
virtue of which they are ascribed. Now in the transitive
process of science such natures may come to be
qualitatively described. When this happens it will of
course initiate a search for the higher-order entities
and mechanisms that account for them. But how does it
happen ?
In general, at any one level, individuals must be
identified and their normic behaviour described. Now for
a qualitative description of a thing or a dispositional
account of its behaviour it must be present to the
scientist's senses and he must be able to describe it
correctly, i.e. as being of the kind or type that it is.
This will normally depend upon two kinds of work:
practical (experimental and technical) work, in which the
scientist's causal and perceptual powers are augmented
(the latter with the aid of the construction of
sense-extending equipment, such as microscopes); and
theoretical work, in which the scientist's conceptual and
descriptive powers are augmented. It is the aim of the
former to produce the object, i.e. to render the thing or
behaviour directly accessible to the scientist's senses
(so that it becomes the possible object of an act of
immediate demonstrative reference). And it is the aim of
the latter to produce the concept of the object, so that
the scientist is capable of an adequate description of
it. Both are necessary for a qualitative description.
It should be noted that the two kinds of criteria,
viz. demonstrative and recognitive, are distinct. For
my incapacity to identify the chromosome structure by
peering down an electron-microscope does not mean that it
is not a possible object of an act of demonstrative
reference. It is present to my senses, whether I
recognize it or not. Conversely to render it accessible
to my senses is an independent labour (itself only
possible if some concept of it is possessed), requiring
great ingenuity, just as experimental production and
control does, when we are concerned with the description
of the law-like behaviour of some thing already
identified. The production of the object and the
production of its concept are thus independent tasks,
each essential to a qualitative description of a thing or
account of its behaviour. The thing must be there and I
must know what kind of thing it is, i.e. how to describe
it; in general this will involve a theoretical
redescription of it.
Now it is important to realize that though the
production of
The Logic of Scientific Discovery 179
the object and the production of its concept are
distinct, the judgement that the object has been produced
itself depends upon a tacit theory of vision and the
instruments according to which its range is extended.
The case of the electron microscope illustrates this very
well. In general it is the function of such background or
auxiliary theories to specify the conditions under which
an object of the appropriate type may be said to be
present to the senses. In this sense they constitute, as
it were, the criteriology of empirical science.
It is clearly essential to the theory of scientific
development proposed here that imagined entities may come
to be established as real. Now an entity may be
`theoretical' either in the sense that its existence is
open to doubt (theoretical_1) or in the sense that it
cannot be directly perceived, either unaided or with the
help of sense-extending equipment (theoretical_2). The
same distinction applies in the case of behaviour. Now
an entity (or mode of behaviour) may be theoretical_1 at
t_1 and perceived and adequately described at t_2, so
that it then ipso facto ceases to be theoretical_1. The
existence of bacteria, initially conceived as minute
hostile micro-organisms, and molecules, initially
modelled on material objects, came to be established in
this way. This is typical of science and shows once more
the importance of viewing it as a process in motion.
But if an entity cannot be perceived, i.e. is
theoretical_2, does this mean that it cannot be known to
exist, so that it must be theoretical_1? If this were
the case all theoretical_2 entities would indeed be
hypothetical, and our knowledge would be necessarily
confied to the domain of observable things, even if this
were now regarded as an expanding class. Fortunately
this conclusion does not follow. For theoretical_2
entities may be known to exist indirectly, viz. through
the perception of their effects. The paradigm here is
the case of the detection of radio-active materials by a
geiger counter, of electricity by an electroscope, of a
magnetic field by a compass needle. That there is a
difference between the cases of detection and perception
is clear. In the case of detection the thing can be
individuated only indirectly, i.e. via the
spatio-temporal framework or through its effects on
particular things; it cannot be the object of an act of
immediate demonstrative reference. Whatever the mental
imagery we use to think of a magnetic field it can be
present to us only through
180 A Realist Theory of Science
its effects. On the other hand my incapacity to identify
a bacterium under a microscope as being of a particular
type, or even as being a bacterium at all, does not mean
that it is not present to my senses; and so capable of
functioning as the object of a possible act of immediate
demonstrative reference, although ex hypothesi I am
incapable of intentionally performing it.
It should be stressed that in the detection case
that something does exist producing the effect is not in
question. Nor is the fact that it exists and acts
independently of its detection. To say `electricity is
what electricity does'55 is to collapse powers to their
exercise. Electricity is not what electricity does; but
what it can do. The mode of reasoning employed in
inferring the existence of causal agents through the
ostension of their effects is thus perfectly proper.
Hence though it is correct to say that when we cannot
qualitatively describe the cause we know less about it
than when we can (given that in the latter case we know
the thing's causal powers as well) it is not true to say
that there is a cause is less certain. It is just that
in the detection case what we can know about a thing is
limited to its causal powers.
Now there are two possibilities here. One is that
there is a nature, susceptible in principle to a
qualitative description, as yet unknown, which is the
bearer of its causal powers. The other is that the
nature of the thing just is its causal powers, as in the
case of physical field theories. At any moment of time a
science may have to put down its ultimate entities just
as powers to produce effects, e.g. to affect observers
and equipment, possible observers and possible equipment,
material things, in certain ways. About such entities
all the scientist knows is their powers. It always
remains possible that he will be able to achieve a
qualitative description of them, and he must strive to do
so. On the other hand, it is also possible that such
entities are their powers. The scientist can never
dogmatically eliminate one of these alternatives in
advance. If there is a frontier to possible knowledge of
the world the scientist can never know when he has
reached it. But whatever is responsible for the world as
manifest must possess causal powers which are continually
being exercised; it must be co-extensive with space and
continuous with time. It must be structured and complex;
it cannot be atomistic or event-like. The concept of a
field of potential seems
55 See A. J. Ayer, The Fundamental Questions of
Philosophy.
The Logic of Scientific Discovery 181
closest to meeting these requirements.56 However it seems
to me there is no reason in principle why there should
not be strata of fields (of perhaps radically different
kinds), forever unknown to us. It should be noted that
only the identification, not the existence of fields
depends upon the existence of material things in general.
Here again the order of dependence in being is opposite
to the order of dependence of our knowledge of being.
The ontological order is distinct from the epistemic one.
The general thrust of my argument in Chapter 2 was
against reductionism. How does this square with my
emphasis on strata of knowledge? It will be remembered
that I did not deny the possibility of an explanatory
reduction but stressed (a) the need for a well-defined
reductans (so that a reduction could not in general be a
means of acquiring knowledge of a higher-order or less
fundamental stratum); and (b) that a reduction left the
reality of the higher-order entities intact, at least in
as much as they were causal agents capable of acting back
on the materials out of which they are formed (see 2.5
above). It is clear that I was there taking possession
of causal powers, and hence existence in time, as the
most general criterion of reality. There is an asymmetry
between space and time here. For powers must be
possessed and exercised in time, but they need not be
localized at any point in space. Relations, for example,
such as that of spin (in physics) and marriage endure
through time and have causal effects. But they have no
position in space. Now in general a reduction is
possible because the entities in terms of which the
behaviour of the thing is explained occupy a different
volume of space, either larger or (more usually) smaller.
Thus the possibility of a reduction implies in general
that the individuals of the different kinds cannot be
said to occupy the same place at the same time and one
not be part of the other. This gives us a general
criterion which imposes limits on regresses of strata,
i.e. upon the possibility of a sequence of (explanatory)
reductions. For one could define a branch of science as
a series of theories within which this criterion is
satisfied. On it, quantum mechanics and chemistry would
belong to the same branch. But electromagnetism and
mechanics, neurophysiology and
56 See R. Harre and E. H. Madden, `Natural Powers and
Powerful Natures', Philosophy, Vol. 45 (1973),
esp. pp. 223-30.
182 A Realist Theory of Science
psychology and (it will be argued) psychology and
sociology would belong to different branches.
Changes of things are explained in terms of
unchanging things. If there are ultimate entities they
must be unchanging. Atoms have already been disqualified
as possible ultimate entities (see 2.3 above). So
ultimate entities must be powers; that is, individuals
characterized solely by what they can do. For if one
could describe the changing states or conditions in
virtue of which their powers were exercised they could
not be ultimate (unchanging). In the last instance to be
is just to be able to do. But this does not rule out the
possibility of a science of cosmology (which would be
concerned with the distribution in space and
redistribution in time of the ultimate entities) or of
irreducibly historical branches of science in which the
ultimate entities were Aristotelian or even Strawsonian
individuals. The transformation of the principles
governing such things would in general have to be
conceived as conjuncturally determined open systemic
events (see 2.6 above). In this way a complex thing such
as a person (or a society) could come to be the cause of
its own transformation.
Now it is because we are ourselves material things
that our criteria for establishing the reality of things
turn on the capacity of the thing whose existence is in
doubt to bring about (or suffer) changes in its material
constitution or the constitution of some material thing.
Space, for example, might be regarded quite abstractly
just as any system of relations in which objects stand to
one another. And we can conceive the possession and
exercise of causal powers in time in ways, and at levels,
forever unknowable to men. We can never know where we
stand absolutely in the chain of being. Despite this
cosmic incapacity science has succeeded in identifying
strata of reality. Now a scientist never doubts for a
moment that there are reasons for the behaviour he has
identified and described. It is in the search for such
reasons, at a deeper level of reality, at present known
to him only through its effects, that the essence of
scientific discovery lies. This search necessitates the
construction of both new concepts and new tools. But, as
what is produced must possess a material cause, the
scientist stands for his essential task, in two systems
of social relationships, depending necessarily on the
work of others.
The Logic of Scientific Discovery 183
I have argued that the concept of natural necessity
is the concept of a real generative mechanism at work, a
concept which is applicable to the world quite
independently of men. And it is in virtue of their
connection by such a mechanism, of which knowledge may be
attained in the social activity of science, that
necessity is properly ascribed to some but not other
sequences. In paragraph 5 I will analyse and criticize some
objections to this concept of natural necessity and the
related concept of natural kinds. But I want to deal
here with the following basic objection to the account I
have proposed: If, as I have contended, at each stratum
or level of reality an entity is identified and its
behaviour is described what positive advantages does this
account have over the traditional empirical realist ones?
I think it has at least four substantial
advantages. First, it reveals the essential movement of
science. Second, it allows room for the location of a
surplus-element, reflecting a difference independent of
men, in the analysis of law-like statements at any one
level. Third, it alone is capable of sustaining the
ideas of the necessity and universality of laws, which
are necessary for the rationality of theory-construction
and the intelligibility of experimental activity.
Finally, it alone is capable of accommodating the
possibility of the existence of entities and the
necessary phase of the knowledge of entities which cannot
be analysed as substances with qualities, but must be
conceived as powers to produce effects, powers which are
possessed and may be exercised quite independently of
their detection. (Needless to say, these advantages are
not independent of each other.)
Science never stops still for a moment. At whatever
level we look, it always involves something more than the
empirical realist concedes. For example, if we consider
the phase of the identification of a protolaw (which
seems prima facie most susceptible to empirical realist
analysis), we find the categorical clause implicit in a
powers ascription, representing the scientist's
instantaneous response to this situation, indicating his
belief in the existence of a reason, located at the next
highest level of inquiry, for the predicates being
conjoined. Only the powers conceptual system is capable
of giving an account of the internal rationality of
science, by which it moves from knowledge of one stratum
to knowledge of the next, so displaying the actual
184 A Realist Theory of Science
historical development of the sciences as something other
than a sequence of accidents.
Now it is our knowledge of the reasons at Stratum II
for the behaviour at Stratum I that warrants our
designating the behaviour as necessary. But the reasons
for the behaviour at Stratum II cannot be collapsed into
the behaviour at Stratum I or an interpretation or model
of that behaviour consistently with the intelligibility
and rationality (respectively) of theory-construction or
the possibility of empirical test. Nor can such reasons
be glossed simply as more fundamental regularities,57 if
they are to be subject to experimental confirmation (or
corroboration) .
I have already shown in detail that the empirical
realist account of laws, and hence the ontology that
underpins it, is defective. Laws, I have argued, cannot
be interpreted as conjunctions of events, but must be
analysed as tendencies of things. If science is to be
rendered intelligible the world must be seen as one of
persisting things, of differing degrees of structure and
complexity, to which powers and tendencies are ascribed;
it cannot be reconstructed as a world of atomistic events
apprehended in sense-experience. Briefly, to summarize
my account of laws: To invoke a law I must have grounds
for supposing a generative mechanism at work. These
comprise: (a) independent grounds, preferably under
experimentally closed conditions, for the mode of
operation of the mechanism; (b) grounds for the
satisfaction of the antecedent (or stimulus) conditions
for the operation of the mechanism on the particular
occasion in question; and (c) the absence of specific
grounds for supposing a breakdown or transformation of
the mechanism in that case. Generative mechanisms, I
have argued, must be analysed as the ways of acting of
things; and their operations must be understood in terms
of the exercise of tendencies and causal powers.
Tendencies may be possessed unexercised, exercised
unrealized, and realized unperceived (or undetected)
by men.
Finally, the empirical realist cannot deal with the
case of entities which just are their powers or about
which all we know are their powers. He thus rules out
dogmatically, tout court the possibility of a certain
kind of entity and a necessary phase of
57 See e.g. P. Achinstein, Law and Explanation,
pp. 13ff.
The Logic of Scientific Discovery 185
knowledge. In virtue of this he is no more able to make
sense of the frontiers of knowledge, than show the
mechanism by which sciences if it can and when it does,
will advance.
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