Plato in insisting:

One of the most heated debates that troubled the church in the Middle Ages was the question of universals. This question goes back as far as Plato’s Forms. It has to do with the relationship between the abstract and general concepts that we have in our minds (what is the relationship between Chair with a capitol “C” and chair with a small “c”?). And from this, two radical viewpoints emerged, realists and the nominalists.

The realists followed Plato in insisting that each universal is an entity in its own right and exists independently of the individual things that happen to participate in it. An extreme form of realism flourished in the church from the ninth to the twelfth centuries. Among its advocates were John Scotus, Erigena, Anselm and William of Chapeaux. On the opposite side were the nominalists and they held that universals were just names, and therefore, have no objective status apart from that which is fabricated in the mind. Nominalists, such as Gabriel Biel and William of Occam (see O section), said that the individual is the only existing substance. Unfortunately, their treatment of nominalism removed religion almost entirely from the area of reason and made it a matter of faith beyond the comprehension of reason.1