Another feature in Aristotle's Metaphysics that has me continually confused is the entire discussion of substance. For me, the discussion seems redundant, because I don't see a logical difference between the (modern) concept of being and substance. (My idea of substance - again, in the modern sense - is not substance-as-object, but substance-as-physical-phenomena. And the physical phenomena are real things that we can know through immediate sense or scientific observation.)
Finally, thought, with the help of Aquinas (Commentary on Aristotle's Metaphysics 434), I am better grasping substance as Aristotle means it: substance is that on which we predicate qualities and spacial attribute. Furthermore, matter is an item that is left over when all of the sensible attributes have been removed from a given substance. It is hard to understand Aristotle's conception of matter in light of modern physics (I'm sure anyone reading this 100 years from now will laugh at modern) because for us, there is no such thing as matter without sensible attribute. For Aristotle, though, matter appears to be an empty shell that, when joined with form, becomes something sensible.
The following lines helped make all of this clear:
"For if the other attributes, which clearly are not substance, are taken away from sensible bodies, in which substance is clearly apparent, it seems that the only thing which remains is matter." (Aquinas, Commentary on Aristotle's Metaphysics, 1281 or p. 434)
