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The categorical irreducibility of dispositions was seen clearly by Aristotle and Leibniz, as discussed by Leclerc , and has been explained at some length recently by Ushenko , Weissmann , Mellor , Harré , Harré & Madden , Emmet  and Franklin , among others (see Tuomela ). (According to Shoemaker , the continued identity of objects also depends on their causal properties.) Opposing philosophical views, however, have been advocated by Locke and Hume, and more recently by Armstrong ([1968, 1969]), Mackie  and E. Prior .
The scientific case about their reality (or not) has been obscured by a lack of any realist interpretations of physical theories, and by an emphasis instead on the mathematical structures and making predictions. Classical and quantum physics will be discussed in more detail in chapters 3 and 4, so is this chapter we will consider more general philosophical objections to the argument that dispositions can never be removed entirely.
There are four kinds of arguments which have been used against the ultimate reality of dispositions.
The reply to this objection has already been outlined: it is in fact not possible to explain dispositions in terms of static structures and actual behaviour. We do need something additional to describe what would happen to the constituents if certain conditions were realised, even if they never in fact occur. It is an illusion that scientific explanations give explanations of dispositions in terms of features which are not themselves dispositional in some definite sense. Mackie  might say that `it is generally believed that what makes the difference between a fragile glass and one that has been toughened by heat treatment is a change in the molecular structure', but a mere rearrangement of a molecular structure will not make any difference unless the inter-molecular forces are thereby conditioned to act differently.
The difficulty with his first argument is that it is not at all clear what `actual' and `categorical' must mean in this context. As we shall find out in chapter 6, 'actual' has meanings ranging from `existing purely actually, excluding all possibilities' through `existing presently' to (in mathematics) `being definite'. It may well be that the sense of `actuality' required for dispositions is the second sense, and the case for this choice will be argued in more detail in chapter 7. We can agree with Armstrong that `it seems that it is impossible that the world should contain anything over and above what is actual. For there is no mean between existence and non-existence'2.6. We do not have to accept, however, that there cannot be actual (i.e. `presently existing') things with something irreducibly dispositional about them. I will be arguing later for the notion of a `partially determinate' particular thing that, while definitely (i.e. actually) existing, has aspects both of actuality and potentiality.
Armstrong's second argument (about `responsibility') will be dealt with below, along with the arguments of Prior . I should also point out that in a later paper2.7, he realises that his arguments for the existence of bases of dispositions do not touch on the `deep question' of whether these bases themselves are still fundamentally dispositional or not.
Mackie's view has recently been supported by Prior , who also claims that a disposition must ultimately be reducible to a non-dispositional basis. She claims for example that, since the laws of physics are contingent, we can imagine possible worlds where, say, a body with inertial mass would not experience finite acceleration under any applied force (i.e. it would not satisfy the usual subjunctive for inertial mass). This would appear to indicate that inertial mass could not be identified with the dispositional property of being such that the subjunctive is always true, but must instead have (or be) a non-dispositional basis. In this latter account, inertial mass is whichever property is responsible (in just this world, or in all worlds) for being such that the subjunctive conditional holds.
The difficulty with this argument is that I cannot see how, in any possible world, a purely non-dispositional basis can ever be responsible for a dispositional property, in the sense of implying the disposition. The (very weak) sense of `responsibility' she has invoked is practically `by stipulation', this being apparently the nature of physical contingency. That is, physical law `just says' that a certain static property called inertial mass is (somehow) `responsible' for a certain subjunctive condition. This however appears to make physical law essentially arbitrary, and makes realistic interpretations difficult. I believe that a more satisfactory account of physical contingency is given by Maxwell's [1968, 1985] `conjectural essentialism' (see also Harré & Madden ). In this account, dispositional properties such as inertial mass necessarily have their associated conditional property, but it is a completely contingent and empirical question whether any given body (or any body at all) has that kind of inertial mass. Similarly, while the sleep-inducing dispositions necessarily induce sleep, it is a completely contingent and open empirical question whether the lump of powder being examined in fact has that disposition. There may be a necessitation (whether logical or natural) between the disposition and its associated conditional, but that does not mean that there is any logical (or natural) necessity between the separate existences of this lump of powder and that later period of sleep.
Maxwell's account (by having bases that are intrinsically dispositional themselves) gives fundamental bases a much stronger sense of `responsibility' for observed dispositions. It is precisely because there are such weak connections between purely static and dispositional properties that the purely non-dispositional bases of Armstrong, Mackie or Prior are unsatisfactory, as dispositions cannot be properly explained by static properties. Inertial mass, for example, must be either implied by or identical with a subset of the basic and fundamental dispositional properties.
Mackie's claim that a description in terms of real powers `usurps any merely contingent explanation' pinpoints a perennial tension in theoretical physics. Apart from the issue of contingency, which we have dealt with above, Mackie's problem is related to the continual question whether the alleged `smallest atoms' discovered in nature are really the ultimate constituents, or whether they have some `merely contingent explanation' in terms of yet-smaller components. This is a real problem, and will not go away merely by stipulating that explanations in terms of parts are always to be found. Perhaps we have found the ultimate constituents in nature. Imagine the dismay that would greet today the claim that the quarks and leptons of modern particle physics, which are presently thought to be fundamental, were really composites of some kind! Such decompositions2.8 are of course not impossible, but the evidence for them must be judged on its merits, not according to preconceived rules.
That most of the simple ideas that make up our complex ideas of substances, when truly considered, are only powers, however we are apt to take them for positive qualities: v.g. the greatest part of the ideas that make our complex idea of gold are yellowness, great weight, ductility, fusibility, and solubility in aqua regia, etc., all united together in an unknown substratum; all which ideas are nothing else but so many relations to other substances, and are not really in the gold, considered barely in itself, though they depend on those real and primary qualities of its internal constitution, whereby it has a fitness differently to operate and be operated on by several other substances.This is the most serious objection of the four, because although Locke would also want to uphold the first two as well, this objection attacks the coherence of the idea I wish to maintain, that dispositions can be intrinsic properties. In order to answer this objection, we require a significant reformulation of what is meant by `power' or `disposition'. We must avoid seeing a power (etc.) as merely a relation between an object and a later event, or between two objects. Rather, as I will show in chapter 7, we must construct a coherent non-relational theory of potentialities. This can be done by first not having the ascription of powers and dispositions refer to the actual outcome or manifestation: this is the real reason for the phrase `C depending on P and the character of A', rather than on the event A itself. Similarly, power conditionals need not refer directly to other particular objects, as Locke thinks they do, if we refer merely to the kind of circumstances for manifestation, not to the actual circumstances. Finally, as will be seen in more detail in chapter 7, we take powers to be the (present) `source' or `means of producing' the outcome event, and not merely constituted by relations to such events. These reformulations result in a dispositional category of existence, one that is not reducible to relations or any other categories, and one that can be `contained in the real existence of things'.
I argue, in conclusion, that dispositional properties can only be explained or reduced to combinations of other dispositions and structures, not to entirely static or structural properties. That is, dispositions have a `categorical irreducibility', as it is impossible to explain them away in terms of other categories such as space, time, form, process, material, property etc. For suppose that the exact shape and size of an object were known, the shapes and sizes of all its constituents, along with a list of these facts at every time. We would still know nothing about how or why the object would change with time or on interactions. Still less could we predict how it would respond to a new experimental test. In fact, if it and its parts had no dispositional properties, as Hume would argue, then we would have his conclusion that any actions or changes (apart perhaps from uniform motion) would be entirely inexplicable: there would be nothing about the object that could lead to these changes rather than to any others. I am not claiming (as does Popper ) that `all physical (and psychological) properties are dispositional', only that there is some irreducibly dispositional component to the reality of nature. ``What there is must be intrinsically related to all the things it would do'' (contra Mackie), otherwise science becomes schizophrenic, with an unbridgeable chasm between its high lands of description and prediction.
Next: 3. Problems in Classical Up: 2. Dispositions Previous: 2.5 Are Bases ultimately Prof Ian Thompson