These terms describe the two schools of medieval logic. Nominalism (from the word Latin word nomen, 'word') argued that the words we use to describe things are mere labels for individual items to which they collectively apply; they have no corresponding reality in or outside the mind. Realistism, on the other hand, held that the individual items are expressions of a thing or concept represented by the word we use of them.
The example most often used to illustrate the difference between them is the sheep. Realists believed that there was such a thing as 'sheepness' of which the individual animals that we call 'sheep' are only imperfect examples. Nominalists believed that 'sheep' is simply individual creatures that we collectively and arbitrarily call by that name; 'sheep' has no reality other than the animal that we see, count, or slaughter.
By rejecting generalizations such as the realists did, nominalists tended to be more empirical and to reject rationalization from first principles.
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