In these four steps you have the whole history of the Western world. Now the man who is mostly responsible for the putting beside it, is Duns Scotus. But I discussed him already in connection with the doctrine of the will and the arguments for the existence of God. But I want to go now directly to the man of whom I spoke very often and whose philosophy I often mentioned, who is in some way the spiritual father of all of you: William Ockham (or Occam), the father of nominalism.
Let me say a little more about what nominalism means. We discussed it in the big survey of the Middle Ages, but we did not discuss it in a detailed way. This fight between nominalism and realism is the destiny of the Middle Ages and largely the destiny of our own time. In our own time it is repeated, partly at least, as a discussion or a fight between idealism and realism, whereby “realism” today is what “nominalism” is in the Middle Ages, and “idealism” today is what “realism” was in the Middle Ages. So here again you must be very cautious about the words. When I speak of medieval realism, I usually add the adjective “mystical” realism.
Now if you hear this word, you are immediately terrified, of course, and don’t think of the modern, sound realism of empiricists and other good people! – they all are based on nominalism in the Middle Ages. What is this nominalism? Ockham criticized the mystical realism of the Middle Ages which thinks the universals are real, in saying that the universals, if existing independently, are special things. If they exist otherwise, they simply reduplicate the things. If they exist in the mind only, they are not real things. Therefore realism is nonsense. Realism which thinks that the universals are real, has no meaning because realism cannot say what kind of reality the universals have. What kin d of reality has “treehood”? Ockham says it is only in the mind, therefore it has no reality at all, it is something which is meant, but it is not a reality. The realists of that time said: No, the universal, “treehood”, which directs every tree in a special direction, is a power of being in itself. It is not a thing – no realist ever said that – but it is a power of being. The nominalists said there are only individual things and nothing else. It is against the principle of economy in thinking, not to augment the principles. If you can explain something like the universals in the simplest term, that they are meant by the mind, then you should not establish a heaven of ideas as Plato did.