Medieval realism is almost 180 degrees the opposite of what we call realism today, and realism today is almost identical with what the medieval people called nominalism. Now this is very confusing, but you as people who have to learn these things should at least be able to understand this confusion.
The reason for it is the following: For medieval man, the universals, the essences, the nature of things, the nature of truth, the nature of man, are powers which determine what every individual tree or every individual man always will become when he or it develops. This is, if you want, mystical realism or, if you want, idealism. Universalia realia – this is medieval realism. They are not, of course, things in time and space; that is a misunderstanding, and then it is a little too easy to reject them and say, “I have never seen “manhood,” I have only seen “Paul” and “Peter”.
Of course this is a wisdom the medieval people, also, knew. But they said all Pauls and Peters always have a nose and eyes and feet and language – this is a phenomenon which must be understood, and it can be understood only if it is understood in terms of the universal, the power of being which we call manhood, and which makes it possible for every man again to become a man, with all these potentialities – which may not develop, which may be destroyed; but he has these potentialities. That is what realism means.
Nominalism is the opposite position which says: only. Peter and Paul, only this tree, at Riverside Drive, at the corner of 116th (the big one there!): that alone exists, and not “treehood,” not the power of treehood, which makes it become one and which makes all the small ones develop – if the boys don’t destroy them! Here you have an example of the difference in feeling. If you look at a tree, you can feel nominalistically and say, “This is a real thing; if I run against it, I will hurt my head.” But you also can look at it and can be astonished, that of all the tree-seeds thrown into the soil, always this structure, shooting up and spreading its branches, etc., develops. And if you do this, then you can see in this big tree “treehood,” and not just a big tree. And in Peter and Paul, you can see not only these particular individuals, but also the nature of man, manhood, as a power which makes it possible that all men have this character.