OK. I look at a tree. I apprehend a tree. How do I know that I really apprehend that tree as it really is? Locke supposed that the knower creates a "little picture" in his mind of the thing known, in this case the tree. But there is a gap between the tree "out there" and the image of the tree in my mind, and there exists no objective ground from which to determine whether the image in my mind corresponds to the tree outside of my mind. This epistemological problem is insurmountable for materialist philosophy.
Aristotle and Aquinas' view was much different. They believed that a tree was a compound substance, that is that it was a thing composed of form and matter. The form can be considered as the non-physical organizing principle of the tree while the matter can be conceived of that which makes a tree this tree.
In the act of knowing, Aristotle and Aquinas would say that the form of the tree becomes one with the mind of the knower (without diminishing or destroying either), thus eliminating the gap between the knower and thing known. Thus we can know with certainty that "we know that tree."
To understand this fully you need to know a lot of Aristotelian terminology like act and passive, the four causes, and accident and substance. You can get a good background briefly at www.aquinasonline.org.
Well, you've never read Rand's "Introduction to Objectivist Epistemology" because -- in that work -- she improves substantially upon Aristotle's work, and what you seem to have learned from Aristotle can be found in Rand's epistemology.