Don't know because he only spoke very briefly after the four had finished. But perhaps the problem he was having, if it was a problem, can be gleaned from a line in Gotthelf's speech. He wrote, "...it isn't clear that Aristotle even has a theory of concepts, or, for that matter, even a technical term that one could reasonable translate as 'concept'" I think Cooper was saying that given what he had heard that day about Rand's thought and given what he knows about Aristotle, they don't have much in common. Maybe we read what we want back into Aristotle. I'm just not sure. Too bad Cooper won't be publishing more on this topic. Perhaps he was focusing on the fact that, as Rand puts it, for her essences are epistemological while for Aristotle they're metaphysical so that in a sense they are not even in the same league, let alone in the same ballpark.