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The process derivation of `continuants' has the feature that in it we can see more clearly how the nature of a continuant (as a propensity field) can be identical with the `particular form of all its powers'. This is because, as was seen just above, all the powers of a propensity field are given by its `substantial form': the form of the field as an extensive distribution of propensity. This is in broad agreement with Ducasse's  account of how a substant is related to its capacities. He argues that
contrary to what the etymology of `substant' may suggest, the relation between a substant and its capacities it `has' is not analogous to the relation between, for example, a table and the objects it `stands under' and `supports'. Rather, the relation between a substant and its capacities is analogous to that which obtains between, for instance an automobile and its parts; or a living body and its organs; or more generally between any whole and its parts.Now, on the present account, a propensity field is a single whole particular thing, and has various possibilities for actualising contained within its extent because it extends and endures (by definition) over all the places possible. One can regard the relation between a propensity field and the places possible within it, or equivalently between a continuant and the interactions possible for it, as therefore just the relation between a unitary whole and the parts into which it may possibly (not actually!) be divided. One important consequence of this account of the continuant as a `whole' with respect to its powers as `parts' means that continuants cannot ever be properly conceived apart from their powers. Thus there never exists any separable, pure or `naked' substance.
The only qualification I would give to Ducasse's account is to note that the actings of a continuant are most often interactions with other continuants, so that an account of a continuant's powers -- what it is capable of doing and how it is capable of interacting -- must make some reference to the condition of the other continuants with which it reciprocally interacts, and not depend only upon its own substantial form.
This substantial form can be regarded as a predicate qualifying `propensity-as-such', as it is propensity (as such) which has that form. Propensity, therefore, can be regarded as the underlying `substance' or `matter' of all enduring continuants, which are therefore `forms of propensity'. `Propensity' is thus the logical subject -- `that which is not predicated of something else' -- and the substantial form is a predicate qualifying this subject.
Propensity is capable of being a subject or a substance, because propensities are kinds of potentialities, and because of the way we resolved the problem of change in chapter 7. There, we saw that in order for potentialities to produced actualities, they had to have some kind of being themselves. That is, potentialities had to exist as `things' just as much as actualities were assumed to do. The alternative to this position would have to have `potential objects' waiting in some kind of limbo before some of them changed to be fully actual. We rejected any kind of `subsistence' like this, and instead we hold that various kinds of potentialities themselves exist as things. That of which they are formed, namely the potentiality itself, is therefore the substance or logical subject of the existing thing.
Traditionally, following Aristotle, this underlying subject is called the matter out of which natural things are constituted. I will not be using this term, as today it leads too readily to the concept of `material substance' of Boyle, Locke and Newton. As I wish to have a concept of substance which is to some extent independent of classical physics, the term `matter' will be used as little as possible.
There are somewhat similar problems in the philosophy of nature. Although we have characterised the substance or the `subject' of things as propensity, and have characterised propensity as the probabilistic potentiality for actuality (and for nothing else), it is still left uncertain what propensity is in itself. We certainly know the forms in which propensity appears, for these are the field structures of mathematics, but I have not said what it is.
The best answer to this is that we must content ourselves with knowledge of everything that a propensity does, and then to take a propensity as the minimal `that which is necessary to do all these things'. Thus propensities, and the substances they form, are defined as the unity of all their powers of operation. There is no reference to anything hidden that might be there as well as this unity: all the substance is `up front' in the capacities for operations. We thus have a kind of operationalism, but with the modal `possibilities for operations' taken realistically.
In any case, if substances are forms of propensities, when we observe substances it must be these propensities which we observe. Of course, we do not observe them as propensities in themselves: we only observe the propensities as they produce effects. It is not the case that we only perceive effects, as the effects are interaction events, and hence are our acts of perception, not the objects of our perception. The objects of our perception are the propensities at the moment when they produce effects. There must therefore be a sense whereby we can say that a propensity is `in' its effects, as during its effects, what we do observe is the substance constituted by that propensity.
It should always be remembered that these `real essences' are always the real essences of particular things. While we can intellectually distinguish the idea of the essence or form of a particular from the idea of that particular thing, that does not mean that the essence or form actually exists apart from that particular object in the world. The real essence is present only so long as the object continues to exist. They are not the `essences' of the medieval neo-Aristoteleanism heavily tinged with neo-Platonism, of which a certain number were supposed, `according to which all natural things are made and wherein they do exactly every one of them partake, and so become this or that species', as Locke 9.10 described the notion. The idea in the present enquiry is not of any such `natural essences' apart from particulars, but of the (essential) natures of particulars. We want to describe the nature which includes what a particular thing is, the principle of any changes it may go through, and that by which it may be intelligible to us.
This is perhaps unconventional, even odd to some extent, but in no other way, I argue, can changes and actualising be intelligibly comprehended. This view of actualising also makes it advisable that there be some indeterminism (no matter how small), in order to distinguish actualities from potentialities. If there is some indeterminism, then actualities (as fully determinate particulars) can be distinguished from potentialities (as partially determinate particular fields). Determinism could perhaps be accommodated, by introducing some other distinction between actualities and potentialities, but that would be an unwelcome complication to the story.
Next: 10. Quantum Substances Up: 9. A Theory of Previous: 9.3 Continuants (Substances) Which Prof Ian Thompson