Whole-Dependent Parts, Aristotelian Organisms, and Causation
In Aristotelian metaphysics, some parts depend on their wholes, at least in a certain respect. For example, the cells of a cat are the cells of a cat. They derive their nature from the whole (reverse mereological essentialism), which means they are not independent of the whole. In fact, an Aristotelian would likely assert that, however far down you go—whether to particles or whatever—you never arrive at a part of an actual substance that is independent of the whole. (For how this fits with modern science, see the book I recommended from Gorman last week.)
Does this mean, however, that composite objects as such don’t require a cause or deeper explanation because there are instances where wholes are more fundamental than their parts? No, of course not. Things can have mutual dependence relations. While dependence is asymtetic, two things can depend on each other in different respects. For the Aristotelian, the reason the cat’s cells have the integrative functional unity they do is because of a more fundamental metaphysical part—namely, form. The entire composite—the hylomorphic composite, the existing cat—depends not just on form but on both form and matter. Form is the principle of organization (that which configures); matter is the principle of individuation (that which is configured).
However, the form isn’t produced by the organism, even if some of its cells are. The whole organism crucially depends upon (and is metaphysically posterior to) its form and matter, so it cannot possibly be the producer or unifier of them, since it only exists when the form is individuated by matter, and this remains true throughout its entire existence. Form and matter themselves sit in a mutual dependence relation, whereas the whole is entirely dependent upon these parts coming together and completing each other with respect to organization or individuation. Since the whole depends upon these parts coming together, it cannot be the cause or explanation thereof without circularity.
All this is related to a heavily discussed grounding issue in Aristotelianism, which I believe ultimately forces one to either 1) reject Aristotelianism in favor of some form of Platonism or 2) consider that all composite Aristotelian substances require a cause as a necessary accident.1
I much prefer option 2.
Why a necessary accident and not an essential feature? Simply because a cat is what it is in virtue of its feline essence. It is not what it is in virtue of being caused. Nevertheless, it cannot exist without being caused. As Gorman points out (pg. 28 in his book), this dependence is not an element of his nature, but rather something that belongs to him on account of his nature or rather is a result thereof. Hence, a necessary accident.