1. The Riddle of Change. Change is ubiquitous. It is perhaps the most pervasive feature of our experience and of the objects of our experience. But is it intelligible? Change could be a fact without being intelligible. But the mind seeks intelligibility; hence it seeks to render change intelligible to it.
There is something puzzling about change inasmuch as it seems to imply a contradiction. When a thing changes, it becomes different than what it was. But unless it also remains the same, we cannot speak, as we do, of one thing changing. But how can this one thing be both the same and different? We ought not assume that there is an insoluble problem here. But we also ought not assume that a simple solution is at hand, or that some simple fallacy has been committed. We must investigate. We do well to begin with some mundane, Moorean fact.
Thus there is an apparent contradiction: the ripe avocado is both the same and not the same as the unripe avocado. If you think this puzzle is easily solved, then you haven't understood it. You cannot understand philosophy unless you understand the problems to which its theories and theses are the responses. Philosophy may be more than aporetics, but it is at least aporetics. If you lack the 'aporetic sense' then you lack the "feeling of the philosopher" Plato describes at Theaetetus 155.
2. Reality is contradiction-free. There are no contradictions in reality, but only in our thoughts about reality. That there are no contradictions in reality is not perfectly obvious, but it is very reasonably assumed to be true, and we have to start somewhere. Now if it is true, and if, as is obvious, changes occur, then the apparent contradiction has to be removed. If the apparent contradiction cannot be removed, that is, shown to be merely apparent, then change, which obviously exists in some sense, will have to be demoted to the status of a Bradleyan appearance. (See F. H. Bradley, Appearance and Reality, Book I, Chapter 5.)
The task, then, is to remove the apparent contradiction.
3. Avocado A, having altered, is both the same and different. How is that possible? One solution involves a distinction between numerical and qualitative sameness. A is numerically the same over the interval during which it goes from unripe to ripe, but not qualitatively the same over that interval. The sense in which A is the same is not the same as the sense in which A is not the same: A is numerically the same over the interval but not qualitatively the same over the interval. I will say no more about this solution in this post.
4. Instead of distinguishing two senses of 'same,' one can distinguish two senses of 'individual.' On an Aristotelian-Thomistic analysis, as presented by John Peterson ("Persons and the Problem of Interaction," The Modern Schoolman LXIII, January 1985, 131-137), the individual1 is the primary substance, the avocado in our example. This primary substance is the individual substance, the individual this-such, together with its accidents. The primary substance at time t (when the ripening process begins) is the unripe avocado, and the primary substance at t* (when the process is completed) is a numerically different primary substance. It follows that the primary substance is not the substratum of change!
The substratum of change — that which stays the same throughout the change — is the individual2, the secondary substance 'in' the individual1. The secondary substance 'in' a primary substance is its individuated essence. Primary substances are capable of independent existence. Secondary substances, as constituents of primary substances, are not capable of independent existence. Yet these secondary substances are what ground diachronic identity. So what grounds identity over time and underlies the change and persists through it is the individuated essence.
One can see how this distinction between two senses of 'individual' removes the contradiction. It is the individual2 (the individuated essence) that is the same over time, and the individual1 that is different over time. There is no one concretum that is the same and different. There are two 'things' in the changing concretum.
It follows that the unripe avocado I saw on my counter a week ago is not numerically the same ripe avocado that I see on my counter today. It is a numerically different avocado.
5. The scholastic analysis can be applied to persons and the problem of their identity over time and through change. Persons are individuals2. Thus they are individuated essences or secondary substances. Elliot drunk and Elliot sober are the same person because the same individuated essence, the same instance of humanity, underlies the change in accidents. But they are numerically different primary substances and thus numerically different human animals.
Peterson thinks this solution superior to the Cartesian view according to which a person is identified with a res cogitans. On the Cartesian view, the person is a primary as opposed to a secondary substance. As a primary substance, the person on the Cartesian view is capable of independent existence. Given that a person remains self-same through change, the res cogitans must remain self-same though change. But then how can Elliot drunk and Elliot sober be the same person? "For individuals1, as was said, are not substrates through any change but are rather the termini in any change, that is, the terminus ad quem [endpoint toward which] and terminus a quo [endpoint from which] of a change." (p. 135)
6. Let us now consider existential or substantial change. When an avocado ripens, it acquires the property of being ripe and loses the property of being unripe. It seems as obvious as anything that alterational or accidental change requires a substratum. But when the ripe avocado becomes the stuff of guacamole, it suffers a much more radical change. The avocado ceases to exist. Can one speak here too of a substratum of change, something in the thing that remains the same through the change?
There is likewise a difference between Elliot's sobering up and Elliot's dying. When he sobers up, the substratum of the change on the Thomistic view is his individuated essence. But what is the substratum of the radical existential change which occurs when he dies? Peterson writes, "The substrate of this change, of course, cannot be a particular human essence since it is just this which has passed away." (135) The particular human essence is subject to passing away since it is a compound of form and matter. So Peterson proposes the Aristotelian view according to which "The substrate of change in the case of essential change is simply the potentiality on the part of any individual to become something essentially different from what it is." (135) This potentiality is matter. Since it is that which individuates the form, matter is an individual in a third sense. There are then three senses of 'individual':
Individual1 = primary substance
Individual2 = secondary substance = substrate of accidental change
Individual3 = matter = substrate of essential or existential change
7. Possible Lines of Critique
A. Peterson's theory implies that the unripe avocado and the ripe avocado are two primary substances, not one. (Likewise for Elliot sober and Elliot drunk.) But since now there is only one, the ripe one, the unripe one must have passed out of existence. This consequence of Peterson's theory seems to collide with the Moorean fact mentioned above, namely, that alterational or accidental change does not involve the destruction of the thing that changes. If the thing that changes is the unripe avocado one sees and touches, then that thing does not remain self-same over time: it passes out of existence. What remains the same over time is the individuated essence which one does not see and which is not a substance in its own right.
B. Matter, as the substrate of essential change, is prime matter. It is very difficult to see how the notion of materia prima can be rendered intelligible.