The primary purpose of this book is to depict the main features of the classical problem of universals in order to provide a better understand- ing of the various suggestions made by the moderns towards the solution of that problem. The work is not historical; however, since knowledge of the history of the problem is essential for understanding the import of the new approach, references are given to classical theories and interpretations are offered without any pretension that they are either exhaustive or final. I have tried to argue that the problem, though often appearing in metaphysical guise, is, in fact, part of the more general problem of semantics, i.e., the relation between words and the world. The medieval theory that universals are the meaning of general words is linked here with a recent theory that the meaning of a word is explainable in terms of its function and not in terms of its putative denotation. What comes of this fusion is this: that if the medieval theory that universals are the meaning of general words is in any way credible, then to know the meaning of such words requires close attention to their functions on the ground that words in general do not mirror the world, rather they are tools used to change it.