Aristotle's 'Metaphysics', Book I, Chp. VI

The selection from the Metaphysics I'm working through skips to chapter 6 after chapter 3, so that's where I'll pick up.

At the beginning of chapter 6 Aristotle makes some contextual/historical points about Plato's philosophy. He first notes that Plato was heavily influenced by the Pythagoreans (the Italian school of philosophy founded by Pythagoras), and by Heraclitus. The Pythagorean influence, Aristotle says, can be seen in Plato's talk of sensible objects "participating" in Forms, just as the Pythagorean school talked of sensible objects "imitating" numbers. He also notes that, just as the Pythagoreans, Plato neglected to ever explain what exactly this relation of "participation" is or entails. The influence of Heraclitus on Plato can be seen in Plato's adoption of Heraclitus' doctrine that "the world (of the senses) is in flux" (think of the classic "you can't step in the same river twice" mantra). Because of this, Heraclitus, and Plato after him, reasoned that you could never have reliable knowledge concerning it.

Next Aristotle begins to actually dig in to Plato's philosophy itself. The first claim is that Plato's ontology essentially boils down to two categories: The "one", and the "great and the small". Here's where, I confess, it started to get tricky for me. I tried to consult Aquinas' commentary on this section, but it didn't end up helping too much (a failing on my part, not Tom's, assuredly). Regardless, the best interpretation I could come up with was something like the following: The "one" in Plato's ontology is the essence of the forms. That is, part of what makes forms (or universals) what they are is that there is only one of each form (although there are multiple instances, or tokens, of those forms). "The great and the small", then, is a term that expresses the essence of particulars. Particulars are, by nature, many. So, for instance, there can be multiple dogs (particular substances), but only one form of Dogness (the eternal form that individuates them).

A third category in Plato's ontology that is briefly discussed is the category of numbers. For Plato, it seems numbers constitute a sort of pseudo-ontological category. They represent (or are generated by) the relation between the "one" and the "great and the small". The individuality (or "oneness") of numbers is found in the forms, whereas their heirarchal structure (construed as a "greater-than" relation) is found in the "great and small" category. ...All that probably seems very convoluted, and for that I apologize. It was, once again, the best interpretation I could grasp with my own lights.

Now Aristotle begins his critique of the Theory of Forms which he will continue on into chapter 9. His first critique is this. Plato says that forms are essentially one, and that particulars are what they are in virtue of their participating in forms. But how can there be only one form per attribute if multiple particulars instantiate them? If Form individuates material substance, then there must be two forms everytime an object acquires its quiddity (this is because form is a constituent of substance for Aristotle).

His next critique is that Plato's theory only addresses the material and formal causes of things. But he neglects, say, their efficient (moving) causes. The Forms only make things what they are in an individuating sense, but they have no power to be the source of the movement that makes some object x an F and then a G. For this, a thing's efficient cause must be invoked.