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The Logic of Scientific Discovery 199 5. OBJECTIONS TO THE ACCOUNT OF NATURAL NECESSITY PROPOSED Having outlined the principle advantages of my account of natural necessity and natural kinds (on pp. 183-5 above), I now want to consider some objections to it. In Chapter 4 I will consider the conditions of the plausibility of these objections. The chief Humean counter-arguments may be put in the form of three theses :- (i) there can be no, or at least no knowledge of, necessary connections between matters of fact; (ii) if there were necessary corrections between matters of fact they would have to be known a priori; so science could not be empirical; (iii) men are never directly aware of any causal power or agency or necessary connections between matters of fact, so these concepts cannot be justified by experience (though they may be explained by it; or are, for the neo-Kantian, imposed upon it). The argument for thesis (i) is typically constructed as follows: there is nothing inconsistent about the supposition that the cause of a phenomenon, say putting a kettle of water on the stove and heating it, should not be accompanied by the effect in question. It is conceivable that water might freeze instead of boil when it is heated. Now thesis (i) is, as stated, highly ambiguous. It is not clear whether it is an ontological or an epistemological thesis (this ambiguity is of course explicit in the way I have formulated it); whether the `necessity' is logical or non-logical; and whether the `matters of fact' are events and states of affairs or the statements describing them. Before returning to the argument, we must see exactly what is at stake in it. 200 A Realist Theory of Science Now, it will be remembered, that for the transcendental realist to say that a sequence E_a.E_b is necessary is to say that there is a generative mechanism at work such that when E_a occurs E_b tends to be produced (is produced in the absence of interfering causes). If there is such a mechanism the sequence is necessary; and its necessity is quite independent of any knowledge of it. To analyse the necessity of the connection in terms of our knowledge of the necessity of the connection would be to commit the epistemic fallacy (see 1.4 above). There is a real difference, quite independent of men, between the fact that when I heat the kettle of water it boils and the fact that it boils when the time is half-past two or the colour of my socks is blue. The necessary connections that bind some but not other events together (which are the enduring mechanisms of nature) are quite independent of our knowledge of them. Statements clearly belong to the epistemic not the ontological order; and logical connections hold only between statements, not between events and states of affairs. Hence the prima facie absurdity of those who, in attempting to refute Hume, try to establish that nomic necessity is, or may be, a species of logical necessity.73 Natural necessity is not logical necessity. Natural connections hold between things, events, states of affairs and the like; logical connections between propositions. Moreover there could be a world without propositions, in which the concept `logical connection' had no application. The laws of logic are not features of the world, nor are they imposed upon it. Rather, we must say: the world is such that changes in it can be consistently described. Neither natural necessity nor knowledge of natural necessity can be identified with logical necessity. But our capacity to deduce the Wiedmann-Franz law from Drude's theory of electrical conductivity may serve as a *criterion* of our knowledge of the necessity the theory describes. I suggested in paragraph 3 above that three levels of knowledge of the objective world order can be 73 See e.g. A. C. Ewing, The Fundamental Questions of Philosophy, pp. 159-81 and B. Blanshard, Reason and Analysis, Chaps. 11-12. Cf. also. N. Maxwell, `Can there be necessary connections between successive events ?', B.J.P.S. Vol. 19 (1967), pp. 1-25; and M. Fisk, `Are there Necessary Connections in Nature?' Philosophy of Science, Vol. 37 (1969), pp. 385-404. The Logic of Scientific Discovery 201 distinguished in the development of science; so that statements can be classified as definitions, deductive consequences of true theories and simple protolaws according to the position they occupy (at any moment of time) in the development of our knowledge. Hence the deducibility of a tendency from a nature may serve as a criterion at the Lockean level for our knowledge of natural necessity, just as a correct definition may serve as a criterion at the Leibnizian level for our knowledge of natural kinds. But whether or not a sequence of events is necessary is quite independent of the logical status of the proposition used to express it; which is a function of the way it is described in the context of our knowledge; which in turn may be shown to have a certain rationale in the development of science. Some causal statements expressing necessary connections are logically necessary and some are logically contingent.74 For the Humean, however, logical and natural necessity are easily confused. For given the isomorphic relationship between knowledge and the world assumed in empirical realism and restricting our knowledge of nature to the protolegal phase of science (see page 172 above) he naturally comes to regard relationships between events as characterizable in the same kind of way as the statements expressing their relationships are at that phase typically, though not invariably, characterized; namely as contingent. But it is into this very same trap that defenders of the entailment view of natural necessity fall. I shall construe thesis (i) as an epistemological claim to the effect that knowledge of necessary connections between events is impossible. And I will attempt to refute it by arguing that unless there were necessary connections between some (but not other) events, science would be impossible; and that in science the most stringent criteria for knowledge of natural necessity may be satisfied. Unless there were necessary connections between matters of fact neither confirmation nor falsification would be possible. For without them no confirmation instance adds any probability whatever to any inductive instance.75 On the other hand for it to 74 Cf. `Tania pushed the door open' logically implies `the door opened'. As Davidson has put it: `the truth of a causal statement depends upon what events are described; its status as analytic or synthetic depends upon how they are described', op. Cit., p. 90. 75. Cf. M. Fisk, op. cit., p. 390. 202 A Realist Theory of Science be rational to reject what is falsified it must be assumed that a hypothesis which has been false in the past will not suddenly become true in the future.76 Whether the conclusions of inductive arguments are weakened to probability judgements or it is denied that science is inductive in nature there must be necessary connections between matters of fact. Such necessary connections are provided by enduring mechanisms. Moreover, if experimental science is to be possible, there must be necessary connections between some but not other events. This implies a dynamic principle of indifference: to the effect that mechanisms not only endure but are transfactually active. Neither their enduring nor their transfactual activity is in need of explanation. Unless there were necessary connections between matters of fact we could have no knowledge, even particular knowledge (in as much as this depends upon inferences beyond what is immediately observed), of the world. For science to be possible then the world must consist of enduring and transfactually active mechanisms; and there must be necessary connections between some but not other matters of fact. Natural mechanisms are of course nothing other than the powers or ways of acting of things. Thus, if science is to be possible, there must be a relationship of natural necessity between what a thing is and what a thing can do; and hence between what a thing is and what it tends to do, in appropriate conditions. The deducibility of a tendency from a nature thus constitutes a criterion for our knowledge of natural necessity. Events are necessarily connected when natural tendencies are realized. With this in mind, let us return to a detailed examination of the argument for thesis (i). Is it conceivable that water should not boil when it is heated? Now it might be said straightaway that it is inconceivable to suppose that water might not boil when it is heated. Since anything that did not boil when it was heated could not properly be said to be `water' at all. That is that, in Lockean terminology, `boiling when heated' specifies part of the nominal essence of water; or we could say with Putnam that `water' functions as a `law-cluster concept'.77 Now 76 Cf. R. Harre, Surrogates for Necessity', p. 380. 77 H. Putnarn, `The Analytic and the Synthetic', Minnesota Studies in the Philosophy of Science, Vol. III, eds. H. Feigl and G. Maxwell, p. 376. The Logic of Scientific Discovery 203 the strength of this reply should not be under-rated. I have no doubt that we should ordinarily say something on these lines. Indeed, unless we have some criteria for the correct application of the term `water' there is no reason why we should use it to refer to substances which as a matter of fact boil when heated rather than to say desk lamps or Saturday afternoons (which do not boil when heated). And such criteria would be at least in part dispositional; appearances, notoriously, can be misleading. Litmus paper that does not turn red when dipped into acid, a metal that does not conduct electricity, or petrol that does not explode when ignited could not be said to be `litmus paper', `a metal' or `petrol' respectively; since the point of referring to the particulars concerned in those ways would be gone.78 A magnet that could not magnetize, a fire that cannot burn or a pen that can never write would not be `magnets', `fires' and `pens' at all. Things must satisfy certain criteria for them to be (correctly identified as) the kinds of things they are. By far the most important of such criteria are those that depend upon their powers to affect other bodies (a class which may be extended, analytically, to include their powers to affect observers under specified conditions in certain standard ways). Such a reply will not however satisfy the Humean (particularly if he believes that definitions are merely matters of convention and cannot express empirically ascertained truths about kinds of things). More to the point it will not satisfy the scientist: for, accepting that `boiling when heated' specifies part of the nominal essence of water, i.e. the criteria for the identification of a substance as `water', he will want to know what it is about water in virtue of which it boils when it is heated. That is, he will set out to construct an explanation, in terms of the molecular and atomic structure of water, from which he can deduce its tendency to boil when it is heated. Now it is clearly inconsistent with this explanation to suppose that water might freeze, blush shyly or do anything else rather than boil when it is heated. That is, if the explanation is correct water must boil when it is heated. Suppose however we came across a stuff which in all other respects looked and behaved like water but which did not boil 78 Cf. E. H. Madden, `Hume and the Fiery Furnace', Philosophy of Science 1971, p. 66. 204 A Realist Theory of Science when it was heated. Assuming standard conditions and a closed system (so as to eliminate the possibility of intervening causes) it would seem that we have the following alternatives: (a) our explanation was false; (b) the fact that it was intended to explain, viz. that water boils when heated, was false (c) the particular concerned had been wrongly identified: it was not a sample of water after all; (d) the particular concerned had changed; so that it had ceased to be water by the time it was heated. Now the Humean asks us to imagine, and inductive scepticism requires that it be possible, that the cause event occurs and the effect event fails to materialize. Let me call this the critical situation. Now I want to argue that, given only that possibility (a) is ruled out, so that we have a correct explanation, the critical situation is impossible; that is, it is not possible that the cause event occurs and the effect event fails to materialize - in our example, that water is heated and does not boil. Let me show this. If the explanation is correct water must boil rather than freeze when it is heated (though of course the converse is not the case); so possibility (b) is ruled out. Consider (c), the misidentification of the particular concerned. Now in this case it is not true to say that water did not boil when it was heated. For what did not boil was not water but only something which looked, and perhaps otherwise behaved, like it, say `nwater'. Finally consider (d), a change in the particular concerned: what was water when it was put into the kettle at time t_1, ceased to be water by the time it froze at t_2. Here again it is not true to say that water did not boil when it was heated. For by the time it froze it had become something else, say `retaw'. Hence given only the possibility of a realist interpretation of the entities postulated in the explanation, the conditions for inductive scepticism cannot be satisfied. If there is a real reason, located in the nature of the stuff, independent of the disposition concerned, water must tend to boil when it is heated (though in an open world any particular prediction may be defeated). The stratification of nature thus provides each science with its own internal inductive warrant Now it might be objected that I have omitted from my list of The Logic of Scientific Discovery 205 alternatives the possibility of the explanation, though correct up to time t_l, subsequently breaking down. But this possibility equally does not satisfy the requirements of the critical case. For, now at Stratum II (defining the Leibnizian level of the particular movement of science with which we are here concerned), nothing which did not possess the molecular and atomic structure that water has been discovered to possess could be said to be `water'. So, here again, it would not be water that was freezing. A stuff remains water only so long as its nature (or real essence) remains unchanged. (Of course scientists could make a taxonomic change, but this does bear upon the argument. It is of course possible that the nature of some particular will be transformed: in which event, scientists will search both for an underlying substance or quasi-substance which preserves material continuity through change (e.g. a gene pool through species change, an atom in chemical reactions, energy in microphysics) and for the agent or mechanism which brought about the change. The principles of substance and causality are interdependent and complementary. Things persist (and continue to act in their normal way) unless acted upon; and their changes are explained in terms of the action of persisting (and transfactually active) things. If science is to be possible changes must be transformations, not replacements; and transformations must be effected by the actions of causes (causal agents). Things cannot pass clean out of existence or events happen for no reason at all. These are ideals of reason. But if science is to be possible our world must be such that they hold. This entails that it must be a world of enduring and continually acting things. It is o course true that it is impossible to prove that cases of ex nihilo production and miracles cannot ever happen. All we can say is that they cannot be known to happen. For it always remains possible for the scientist that what appears to be a case of an ex nihilo production or a miracle at time t_1 can come eventually at t_2 to be explained in terms of the transformation of real things and the action of real causes upon them. I have argued that provided we have a correct explanation the critical situation cannot occur; that, for example, as long as the particular stuff remains water it must tend to boil when heated. But it might be urged if, as I have acknowledged, the nature of 206 A Realist Theory of Science some particular may be changed does this not open the floodgates of inductive scepticism once more? The answer is no: for there is a big difference between wondering whether some particular will be so acted upon by real causes in its environment that its nature (in this case, molecular structure) will be transformed, so that it ceases to be an individual of that kind, and wondering whether, while remaining an individual of that kind, it will cease to behave in the way that it has tended to behave in tr the past. The point is even clearer if we generalise it, so raising the questions of the boundaries of kinds and of the scope of application of laws. The difference is between wondering whether water will cease to exist; and wondering whether, while continuing to exist, it will stop boiling (in exactly the same circumstances) when it is heated. It might be objected that while what I have said clearly covers case (d), viz. that of a particular changing, I have not taken the possibility of case (c), viz. that of a particular being misidentified, of nwater being mistaken for water, seriously enough. What is to prevent us continuously misidentifying particulars in just this way ? Now just as particulars may be transformed, so they may be misidentified. But the situation the inductive sceptic asks us to imagine only gets of the ground if we assume that the relevant particulars have been correctly identified. The problem of induction is the problem of what guarantee we have that the unobserved will resemble the observed, or the future the past; it is not the problem of what guarantee we have that we have correctly observed the observed or correctly described the past. The suggestion that what I have here may in fact be a piece of lead piping is irrelevant to the question of what warrant I have for assuming that water will continue to boil when heated or for supposing that there is a necessary connection between water boiling and its being heated. Nevertheless despite this irrelevance to our present concern scepticism about particular knowledge can and should be met. It might be met in the following way: Any argument in which the case for the general misidentification of particulars is stated itself presupposes the capacity to identify certain particulars, namely words as tokens of a type and hence possessing a certain standard meaning in a given context. Hence no argument for the general misidentification of particulars can be consistently The Logic of Scientific Discovery 207 stated. If this argument does not carry conviction try to imagine a world in which we (a) systematically (b) at random misidentified (alpha) some particulars (beta) all particulars (a') all the time (i) some of the time. A world in which we systematically misidentified some given class of particulars (such as books as saucers and vice versa) would just be a world in which objects had different names. But a world in which our misidentifications were haphazard or universal is not coherently conceivable. It makes no sense to say that a particular has been misidentified unless one is prepared to say in what respect it has been misidentified. This itself presupposes the capacity to identify the particular as of a certain type. Of course our capacity to identify particulars presupposes the extended or dynamic principle of substance enunciated above, namely that things persist and continue to act unless acted upon, and hence in this way it presupposes the existence of necessary connections between matters of fact. It is up to the criteriology of empirical science to determine whether a particular has been misidentified or a perceptual report is nonveridical. The point is, however, that if science it to be an ongoing concern it cannot persistently demand and persistently return negative verdicts. It might be objected to my refutation of thesis (i) that I have not considered the possibility that the explanation, which gives each science at any moment of time its own inductive warrant, is incorrect. Now it is of course always possible that we are mistaken in our explanation of why water must boil when heated; that our description of the mechanism in virtue of which it does so is wrong. But this is a general condition of all knowledge; it does not bear on the argument of thesis (i), which concerns the special difficulty of knowledge of necessary connections between matters of fact. I have already argued against the idea that all knowledge is conjectural on the grounds that refutations presuppose acceptances (progress requires a material cause). But whether or not my account of the transitive dimension of the philosophy of science is accepted, refutations presuppose necessary connections between matters of fact. I have argued that scepticism about change, about our capacity to identify particulars and about the possibility of non-conjectural knowledge as such are all distinct from the special kind of scepticism involved in thesis (i), which is 208 A Realist Theory of Science scepticism about the possibility of knowledge of necessary connections between matters of fact. I have shown how the second and third forms of scepticism, though irrelevant to thesis (i), may be averted. But how can Heraclitean scepticism be countered? Changes in things, I have argued, are explained in terms of unchanging things. The world is stratified. We need only worry about whether atoms will cease to exist when tables and chairs do; we need only worry about whether electrons will cease to exist when atoms do. It is contingent that the world is such that science is possible. But given that it is the dynamic principles of substance and causality that I have formulated must be true of it. Three further forms of Heraclitean scepticism are possible in which we could be invited to imagine that our world is replaced (a) by a totally different one; (b) by one in which the principles of substance and causality no longer held; and (c) by one in which science ceased to be possible. I shall argue that the replacements envisaged in (a) and (b) are impossible, but that I am precluded by my own premises from saying anything about (c). In (a) it is supposed that our world could be replaced by a totally different one; but to which, once it had come into being, inductive techniques could be reapplied. Now this is not an intelligible supposition, not only because scientific continuity would be lost during the replacement (so it would make little sense to talk of reapplying inductive techniques), but because there is no possible way in which such a replacement could be affected save by the action of real causes.79 In (b) it is supposed that our world might be replaced by one to which the principles of substance and causality do not apply. Now although the existence of our world is contingent, given that it exists the supposition that it might be replaced in this way is not an intelligible one. Transcendental realism demands that we reason from the effect, science, to the condition of its possibility, viz. a world of enduring and transfactually active mechanisms. So we can rest assured that long after mankind has perished things will persist and continue to interact in the world that we once lived in. This leaves us with (c), about which I have said 79 It is of course inconceivable that a fundamental entity or entities should act inconsistently with its (their) nature. Hence in the last (non-Laplacean) instance everything is as it must be. The Logic of Scientific Discovery 209 my premises preclude me from speaking. But a moment's reflection shows that (c) is devoid of interest for us. It is an empty counterfactual. For we know as a matter of fact that our world is one in which science is possible. Hence to assert the possibility of a world without science is merely to reassert the contingency of the circumstance that makes a study of the conditions of the possibility of science possible. I have established that we can have (and that science actually possesses) knowledge of necessary connections between matters of fact. And I have shown how inductive scepticism proper, namely that arising from the assumption of the possibility of what I have called the critical situation, viz. the occurrence of the cause event and the non-occurrence of the effect, can be allayed, viz. by the provison of an adequate explanation; and how the other forms of scepticism often confused with inductive scepticism can be countered. I now turn to theses (ii) and (iii) which the Humean uses to bolster his central contention. Thesis (ii) alleges that if there were necessary connections between matters of fact they would have to be known a priori, so that science could not be empirical. It is clear that this argument trades on a tacit conflation of logical and natural necessity and the identification of the resultant concept with that of the a priori. To refute it, I will have to show how knowledge of the natures or real essences of things, which I have argued ground our ascriptions of natural necessity, can come to be attained empirically; that is, how a posteriori knowledge of natural necessity is possible. As there is some misunderstanding about the role of the concept of essence (and, as we shall see, the nature of definition) in science, some preliminary terminological clarification is necessary. The nominal essence of a thing or substance consists of those properties the manifestation of which are necessary for the thing to be correctly identified as one of a certain type. The real essences of things and substances are those structures or constitutions in virtue of which the thing or substance tends to behave the way it does, including manifest the properties that constitute its nominal essence. Science, I have argued, seeks to explain the properties of things identified at any one level of reality by reference to their intrinsic structures, or the structures of which they are an intrinsic part (defining the next level of 210 A Realist Theory of Science inquiry). Thus the dispositional properties of say nickel, e.g. that it is magnetic, malleable, resistant to rust, melts at 1445 degrees C and boils at 2900 degrees C are explained, in the context of post-Daltonian atomic theory, by reference to such facts about its intrinsic structure as that its atomic number is 28, its atomic weight is 58.71 and its density is 8.90. The atomic constitution of nickel is its real essence. But it was discovered a posteriori, in the transitive process of science. And it itself constituted an explanandum of the next phase of scientific inquiry. In general to classify a group of things together in science, to call them by the same name, presupposes that they possess a real essence or nature in common, though it does not presuppose that the real essence or nature is known. Thus we are justified in classifying alsations, terriers and spaniels together as different varieties of the same species dog because we believe that they possess a common genetic constitution which, despite their manifest sensible differences, serves to differentiate them from the members of the species cat. A chemist will classify diamonds, graphite and black carbon together because he believes that they possess a real essence in common, which may be identified as the atomic (or electronic) structure of carbon, of which these are allotropic forms. To classify a thing in a particular way in science is to commit oneself to a certain line of inquiry. Ex ante there will be as many possible lines of inquiry as manifest properties of a thing, but not all will be equally promising. Thus if one's concern is to account for the manifest properties of cucumbers it is clearly preferable to classify a 12 in. long green cucumber under the sortal universal `cucumber' rather than under the universals `green' or `12 in. long'. Not all general terms stand for natural kinds or taxa; because not all general features of the world have a common explanation. Carbon and dogs constitute natural kinds; but tables and chairs, red things and blue, chunks of graphite and fuzzy dogs do not. The justification of our systems of taxonomy, of the ways we classify things, of the nominal essences of things in science thus lies in our belief in their fruitfulness in leading us to explanations in terms of the generative mechanisms contained in their real essences. Not all ways of classifying things are equally promising because not all sets of properties individuate just one and only one kind of thing. The Logic of Scientific Discovery 211 The distinction between real and nominal essences should not be confused with that between real and nominal definitions. Real definitions are definitions of things, substances and concepts; nominal definitions are definitions of words. (Nominal essences are the properties that serve to identify things). Real definitions, in science, are fallible attempts to capture in words the real essences of things which have already been identified (and are known under their nominal essence) at any one stratum of reality. As so conceived, they may be true or false (not just or even - more or less useful). The atomic weight of copper is 63.5. It would be wrong to claim that it was 53.4 or alternatively that 63.5 was the atomic weight of tin. Of course this fact was discovered a posteriori; but it may now be said to constitute part of the real definition of copper. If the real essence of copper consists in its atomic (or electronic) structure, its nominal essence might consist in its being a red sonorous metal, malleable and a good conductor of electricity etc. Something that did not satisfy these properties could not properly be said to be `copper'. But conversely just because the word `copper' in science has a history, and at any moment of time a use, the nominal essence of copper cannot suddenly be designated by the use of `reppoc' or `tin'. Nominal definitions in science cannot therefore be conceived as stipulative, arbitrary or matters of convention. Although there is a sense in which any other symbol could have been used to refer to copper; given this usage and that history `copper' cannot be replaced by `bronze' or `female' for no reason at all.80 Changes in the definitions of words in ongoing social activities require justification. On the view advanced here science consists in a continuing dialectic between taxonomic and explanatory knowledge; knowledge of how the things there are behave. It aims at real definitions of the things and structures of the world as well as statements of their normic behaviour. The source of the failure to see this is the ontology of empirical realism which reduces things to qualities, taxa to classes, enduring and active mechanisms to constant conjunctions of independent and atomistic 80 For a discussion of the history of `copper' see M. Crosland, Historical Studies in the Language of Chemistry and R. Harre and E. H. Madden, Causal Powers, Chap. 1. 212 A Realist Theory of Science qualia.81 Now if the world consists only of qualia and qualia are independent of one another then the particular names that we give to qualia cannot matter and all qualia will appear on a par. On this conception, predicates must be independent of one another and classification is ultimately arbitrary. Now just as it is a mistake to assume that science is concerned with any and all behaviour it is a mistake to assume that it is concerned with any and all things. Scientists do not seek to describe the behaviour of or to classify common objects like tables and chairs, though the laws of physics and the principles of scientific taxonomy (e.g. the identification of a table as an oak one) may be brought to bear on them. Now from the fact that tables have no real essence it does not follow that carbon has none. Electrons are not related in the same way as games. A resemblance theory of universals works best for the complex Strawsonian individuals of ordinary life. But the universals of interest to science are real: they are the generative mechanisms of nature which account, in their complex determination, for the phenomena of the world, including (upon analysis) the genesis and behaviour of ordinary things. The dialectic of explanatory and taxonomic knowledge must thus be formulated as follows: science is concerned with the behaviour of things only in as much as it casts light upon their reasons for acting and hence upon what kinds of things there are; and science is only concerned with things of a particular kind, in as much as they constitute the reason for some pattern of normic behaviour and thus themselves become an appropriate object of inquiry. The importance of taxa in science may be expressed by saying that what is non-accidentally true of a thing is true of a thing in virtue of its essential nature. A thing acts, or at least tends to act the way it is. It should be stressed that the difference between a thing which has the power or tends to behave in a certain way and one which does not is not a difference between what they will do, since it is contingent upon the flux of conditions whether the power is ever manifested or tendency exercised. Rather, it is a difference in what they themselves are; i.e. in their intrinsic natures A copper vase remains malleable even if it is never 81 To use Goodman's very useful term. See N. Goodman, The Structure of Appearance, p. 130 and passim. Goodman himself attributes the term to C. I. Lewis, Mind and the World Order. The Logic of Scientific Discovery 213 pressed out of shape. It is contingent whether an electric current is ever passed through a copper wire. But it is necessary, given its electronic structure, that it be a good conductor of electricity. We know how things will behave, if certain conditions materialize, if we know what the things are. But we can only know what things are a posteriori, via the empirical process of science. This view may be contrasted with the idea that scientists are not concerned with questions such as `what is energy?' or `what is an atom?' but only with questions of the kind `how can the energy of the sun be made useful?' or `under what conditions does an atom radiate light?'82 Popper's `methodological nominalism' seems to be based on the idea that to suppose that things have essences is to suppose that it is possible to give explanations which are `ultimate' in the sense that they are insusceptible in principle of further explanation (which is what he calls `essentialism').83 Although Locke may have held this view, is it certainly no more a necessary feature of the concept of real essence than it is a necessary feature of the concept of behaviour to suppose that because a thing can be described as behaving in a certain way the behaviour itself cannot be subject to further explanation. It is clear that to suppose that things have real essences is not to suppose that the real essences of those things cannot be explained in terms of more fundamental structures and things. Two other arguments sometimes invoked against the concept of real essences should be mentioned. The first depends upon the assumption that differences in nature are continuous, not discrete; that `God makes the spectrum, man makes the pigeonholes';84 so that `genera, species, essences, classes and so on are human creations'.85 I can find no possible warrant for such an assumption. Taken literally, it would imply that a chromosome count is irrelevant in determining the biological sex of an individual, that the class of the living is only conventionally divided from the class of the dead, that the chemical elements reveal a continuous gradation in their properties, that tulips merge into rhododendron bushes and solid objects fade gaseously away into empty space. The second involves the belief 82 K. R. Popper, The Open Society and Its Enemies, Vol. I, p. 32. 83 K. R. Popper, Conjectures and Refutations, Chap. 3, esp. p. 102. 84 A Flew, op. cit., p. 450. 85 Ibid., p. 449. 214 A Realist Theory of Science that to suppose that there are natural kinds is to suppose that these kinds are fixed, and is in particular to rule out the possibility of a mechanism of evolution.86 Again, this is completely unwarranted. For natures may change; and whether, and if so the ways in which they do, are matters for substantive scientific investigation. No spectrum exists between men and apes but that does not preclude the possibility of a mechanism of evolution (involving a whole sequence of `missing links'). What happens in such cases is that biologists posit a novel entity, a gene pool, as the underlying continuant through the species' change. The objection is only valid at the level of ultimate physical entities since necessarily if such entities exist they must be enduring. Scientists attempt to discover the real essences of things a posteriori, and to express their discoveries in real definitions of the natural kinds. From a description of the nature of a thing its behavioural tendencies can be deduced. When such tendencies are realized the events describing the stimulus or releasing conditions for the exercise of the tendency and its realization may be said to be necessarily connected. Thus scientists can come to possess knowledge of necessary connections between events as a result of an a posteriori process of discovery. Scientists are not content to collect conjunctions of events. Rather they try to discover the natures of things. Given this, no problem of induction can arise. Since it is not possible for a thing to act inconsistently with its own nature and remain the kind of thing it is. That is, a thing must tend to act the way it does if it is to be the kind of thing it is. If a thing is a stick of gelignite it must explode if certain conditions materialize. Since anything that did not explode in those circumstances would not be a stick of gelignite but some other substance. Now given the satisfaction of the criteria for the identification of a substance, say water, and the recording, preferably under experimentally closed conditions, of its most significant and suggestive behavioural properties, scientists move immediately to the construction and testing of possible explanations for the protolaws identified. But if there is an explanation, located m the nature of the stuff or the system of which the stuff is a part, whether or not it is known by men, water must tend to boil when it is heated. It is the real 86 S. Toulmin, op. cit., pp. 135-6. The Logic of Scientific Discovery 215 stratification of nature that justifies induction in science. It is not we that impose uniformities upon the world, but nature that makes induction (properly circumscribed) a rational activity for men. The third Humean counter-argument is that we are never directly aware of any necessary connection between matters of fact or causal power or agency so that these concepts cannot be justified by experience. Thesis (iii) thus completes a triangle, whose other sides are theses (ii) and (i). It could be argued that we are sometimes directly aware of necessarily connected sequences (see 2.3 above), and that we are sometimes directly aware of the exercise of causal powers (though the powers themselves can only be known, not shown, to exist; i.e. we are never directly aware of causal powers as such).87 It seems clear that we are aware of ourselves as causal agents in a world of other causal agents; and that unless we were so aware we could not act intentionally, or come to know ourselves as causal agents at all. (Projective explanations of our idea of necessary connection are clearly anthropocentric.) However for the transcendental realist this is incidental. For, for him, the status of the concept of necessary connection is clear: it has been established, by philosophical argument, as applicable to some but not other sequences of events as a necessary condition of the social activity of science. (It should be stressed that this does not mean that any particular science has correctly identified, let alone adequately described, the necessary sequences: it is a condition of the possibility of science.) Thus the concept of natural necessity does not have to be justified in terms of or traced back to its source in sense-experience; though there must be a scientific explanation of how we come to possess the concept. That science has a posteriori knowledge of necessary connections between matters of fact is a proposition that can be given no further justification. 87 E. H. Madden and P. Hare do not clearly distinguish powers from their exercise in their criticism of this Humean argument in `The Powers That Be', Dialogue 1971, pp. 12-31.

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