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To say that salt is soluble in water, that this piece of metal is flexible, that glass is fragile, or that steel is hard, is to ascribe dispositional properties to these things. These are the kinds of things that can be determined by experimental investigations, and are general facts for which we can collect evidence. Often a single trial will be sufficient to determine, say, that salt does dissolve in water when suitably immersed. The meaning of saying that salt is soluble, we should note, goes beyond reporting the results of our few experiments. It is to say that it is a general property of salt that whenever it is placed in water, then it will dissolve. Similarly, to say that this piece of metal is flexible, is to say that whenever it is subject to a transverse pressure, then it will bend. To say that steel is hard is to say that whenever it is subject to a force, then it will be only slightly deformed. To say that glass is fragile is to say that whenever it is suitably hit it will break.
What is important about dispositional properties is this `if then ' feature. Dispositions are thus different from other properties such as place, size and shape which describe only present states of affairs, and make no reference to what might happen in the future. For this and other reasons, there has been considerable debate within the philosophy of science as to the extent and importance of dispositional properties, against what we might call purely static or structural properties.
The issue of dispositional versus static properties will be considered latter in the chapter, for it is important, as Mackie  points out, to separate three different sorts of question: questions about meaning, about what is being said when dispositions are ascribed to things; questions about what we know, about when and how we are able to ascribe dispositions to things; and questions about what is there, about what sorts of properties or states or processes are objectively present in the things that we describe in dispositional terms. To these, I add questions of reality, whether we can completely avoid dispositional terms, and questions of explanation, as to how dispositional properties are in fact explained scientifically.
A great many terms have been used over the years to describe what we call here `dispositions' or `dispositional properties'. In a very general sense, any `ability', `capability' or `capacity' refers to a meaning of this sort, but as these words have meanings far beyond the philosophy of nature, we will not adopt them as technical terms. Aristotle used the term dynamis (potentiality) to refer to the general capability of things to cause changes in others. Locke  used the term `power', and says that fire has a power to melt gold, and gold has a power to be melted; that the sun has a power to blanch wax, and wax a power to be blanched by the sun. We are abundantly furnished with the idea of `passive power', he points out, by almost all sorts of sensible things. More recently, Harré2.1 has revived the word `power' as a general term for scientific explanations, and his powers are either identical with dispositions or closely related to them. In science, terms such as `force' and `potential' (as in `potential energy' or `field potential') have been introduced, and these are all dispositional in an essential manner.
I will be using the terms `power', `potential', `capability', `capacity', `propensity' and `cause' all as examples within the class or category of `dispositional properties of objects'. (There will be more discussion of these different terms in chapter 7.)
Propensities are properties of objects which, in appropriate circumstances, give rise to real and objective probabilities. These probabilities, if they are truly the product of propensities, are not merely an expression of ignorance or partial knowledge on our part. We sometimes still consider dice to have propensities for landing with different numbers upward. As this is only because of our ignorance of their exact trajectories, however, they only have `propensities' in a secondary or `subjective' sense2.2.
Exactly whether and how there can be any real and objective propensities will be discussed below, in the context of that question for dispositions in general. Kyberg  and Maxwell  have more specific discussions of the probabilistic aspects, and give detailed comparisons with the frequency interpretation of probabilities. Humphreys  discusses the question of whether talk of propensities can be replaced by talk of probabilities.
If we want to know what it means when we say that salt is soluble, we will need to know that the `if then ' phrase means. We need to specify both the antecedent condition, the manifesting occurrence, and the logic of the `if then ' expression. In general, the ascription of properties in dispositional category is of the form
Object S has the disposition P to do action A if S is in some circumstance C, C depending on P and the character of A, then there will be a non-zero likelihood of S doing A.Here, the `action A' can either be a change in S itself or an interaction with other objects. The suitable `circumstance C' is usually defined by multiple spatial relations to other objects, and will be different for different dispositions for different actions. The circumstance C is said to depend only on the `character' of the action, and not the action itself, because possibly, if the disposition is never manifested, there may exist no such action, at any time in the past, present or future. The circumstance C should depend only of the kind of event expected, and not on its actual occurrence. Finally, the phrase `non-zero likelihood' is designed to be sufficiently general to allow both sure-fire dispositions and probabilistic propensities. It has the consequence that if the probability of an event (while varying with time) touches zero, then there is no propensity at that particular time, but this is surely a reasonable feature.
Harré and Madden  add a phrase `in virtue of the constitution of S' to the above form, in order to exclude `changes' to certain properties of S that are changes in purely external relations that may come about completely independently of whatever S is actually like. Thus, for example, no disposition of Socrates is necessary to explain his becoming smaller than Theaetetus, if it is the latter who is growing. Mackie  argues however, that it is not part of the logical meaning of a disposition that it is based on some internal form of the objects concerned. He admits that the existence of some basis is an extremely plausible empirical hypothesis, but not that it is logically implied by every ascription of a disposition on all occasions. This question of `basis' will be further discussed below.
As is well known, the logical meaning of the `if then
' expression italicised in the above form of ascription can not
be taken to simply be the material conditional. The meaning of `x is
fragile' -- F(x) -- cannot be defined as
where H(x,t) means `x is hit at time t' and B(x,t) means `x breaks at time t', because this would make fragile every object that was never hit. If we defined F(x,t) to mean x is fragile at time t', then
would make fragile at t every object that was not hit at time t. Mackie  describes a variety of alternative logical expressions designed to capture the required if/then meaning, and how their inadequacies indicate that we need conditionals which are not themselves purely material conditionals. Neither, he and Mellor point out, can dispositional statements involve purely counterfactual conditionals, as then we get the absurd result that salt is no longer soluble once it is dissolved.
D'Espagnat  explains how one could try to use what Carnap called `partial definitions' for the meaning of dispositional terms. This amounts to translating the statement `x is fragile at time t' into a `reduction sentence' of the type
If x is hit at time t, then it is called fragile if and only if it breaks.or, in the language of predicate calculus,
This definition can be verified not to suffer from the deficiences of the previous proposals. On the other hand, d'Espagnat points out, the range of definition (2.3) is obviously much smaller than that of definition (2.2). This is because (2.3) yields no interpretation for a statement such as `object x is fragile and it is not being hit'. If no other partial definitions of `fragility' are then applicable, the statement turns out, to the logician, to remain incomprehensible and even meaningless.
The conclusion both Mackie and d'Espagnat come to is that dispositional
statements are equivalent to some kind of non-material conditional outside
the range of traditional (non-modal) formal logic. They cannot,
furthermore, be identified specifically with counterfactual conditionals.
The if then in
F(x,t) = if H(x,t) then B(x,t)
is therefore non-material: whether the statement would take an open, subjunctive or counterfactual form depends on the circumstances and on the speaker's beliefs about the circumstances. If the glass was known not to be hit at time t, then a counterfactual form is appropriate; if it was known to be hit at time t, then we have simply F(x,t) B(x,t); whereas if it is not known whether it was hit or not, then we use the subjunctive form `if the glass were to be hit, then it would break'.
These implications together amount to what Mackie calls a minimal disposition. The ascription of a minimal disposition is taken as equivalent to the assertion of a suitable non-material conditional, and this encompasses a large part of the everyday meaning of dispositional properties. They allow dispositions to be ascribed both when they are being manifested and when they are not. They allow glass to be fragile for a while, and then to be toughened by heat-treatment and to be fragile no longer. They allow a piece of wrought iron to have a period of brittleness when it was cooled to then temperature of liquid air. They also allow a thing to have a disposition even if neither it or anything else ever manifests that disposition (such as the disposition for a nuclear explosion, to use an example from Mellor ).
Next: 2.2 Derivative Dispositions Up: 2. Dispositions Previous: 2. Dispositions Prof Ian Thompson