It's an old maxim of Aristotelian metaphysics that substances do not have substantial proper parts. Here's an argument for it, in the case of material substances. Suppose a material substance A has a substance B as a proper part. Now, arguably A is wholly composed of two parts: the matter M and the form F.
Now the form G of B cannot overlap M, as then the form would be partly material. So G must be a part of F. But forms of substances are simples. So G must be all of F. But then we have two substances with the numerically same form, and that seems absurd.
A central assumption in the argument is that forms are simples. There may be a way of making an argument without that assumption. Suppose we say that G, the form of B, is a proper part of F, the form of A. Now if any proper part of a substance is a substance, then my heart is a substance--it's nicely delineated, and one of the best candidates. But I can survive the destruction of my heart (I would just need a machine to circulate the blood). And surely if my heart is destroyed, its form is destroyed as well. But my form doesn't seem to be intrinsically changed by the destruction of my heart. Yet if the form of the heart were a part of my form, then my form would be intrinsically changed by the destruction of the heart.
6 comments:
1. Isn't the human soul supposed to be a "substantial form"? I.e. a substance for all practical purposes, and yet part of another substance (the person)? And if not, what is the definition of "substance" such that the human soul is not one?
2. If hearts are parts of bodies, but not essential parts of bodies, maybe forms-of-hearts could be parts of forms-of-persons but not essential parts.
Curious question: Why the restriction to substantial proper parts as opposed to any old proper parts (like the form and the matter)?
Cruz: this argument answers that question. It only works for substantial parts.
Alex: Right. That's a cool result.
So I guess I was curious if this was what originally motivated the Aristotelean maxim. And if not, then do the other motivations undermine a hylomorphic conception of substances?
You are relying on the transitivity of parthood to get that G is a part of A and therefore a part of A's matter, M, or its form, F. But you are equivocating on "part" when you use transitivity of parthood: you infer from the fact that B is an integral part (that is, a part in the sense that contemporary analytic philosophy conceives uses the word "part") of A and the fact that G is a substantial part (that is, form or matter) of B, that G is a substantial part of A, and therefore must be a part of M or F. But that doesn't follow due to the equivocation on "part". So the argument fails.
More on the relevant distinction: http://plato.stanford.edu/entries/mereology-medieval/#3.1
Cruz:
Another motivator for the maxim is this: If a material substance is made of substances, then they have some matter in common. But the form of a substance is what gives form to the matter. So there would be a bit of matter with two forms. And that seems problematic.
Christopher:
It seems to me that both kinds of parts are kinds of parts. And parthood is itself transitive, not just substantial parthood or integral parthood.
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