I'm not very educated. What are "nominalists", when you have the time to explain a little?
Nominalism is an attempt to solve the "the problem of universals." We know both "this cat" and "cat," or "catness." "This cat" is the particular term, "catness" is the universal term. But what does "catness" refer to? Does the term "catness" refer to anything real, or does it exist purely in the mind as a name given to a set of characteristics common to various particulars? If so, how can I know that my understanding of "catness" is the same as yours, since we have diverse experiences with furry creatures? The problem for Nominalists is insoluble, and Nominalism tends to devolve into solipsism or relativism.
Realists, on the other hand, believe that universal terms refer to real natures (essences or forms).
When the mind apprehends the essence of a thing (quod quid est; tò tí en eînai), the external object is perceived ["this cat," in my example] without the particular notes which attach to it in nature (esse in singularibus)[quantity, quality, relation, place, time, position, possession, doing, and undergoing] and it is not yet marked with the attribute of generality which reflection will bestow on it (esse in intellectu). The abstract reality is apprehended with perfect indifference as regards both the individual state without and the universal state within: abstrahit ab utroque esse, secundum quam considerationem considerattur natura lapidis vel cujus cumque alterius, quantum ad ea tantum quæ per se competunt illi naturæ (St Thomas, "Quodlibeta", Q. i, a. 1). Now, what is thus conceived in the absolute state (absolute considerando) is nothing else than the reality incarnate in any give individual ["cat"]: in truth, the reality, represented in my concept of man, is in Socrates or in Plato. There is nothing in the abstract concept that is not applicable to every individual; if the abstract concept is inadequate, because it does not contain the singular notes of each being, it is none the less faithful, or at least its abstract character does not prevent it from corresponding faithfully to the objects existing in nature. As to the universal form of the concept, a moment's consideration shows that it is subsequent to the abstraction and is the fruit of reflection: "ratio speciei accidit naturæ humanæ" ["catness," in my example]. Whence it follows that the universality of the concept as such is the work purely of the intellect: "unde intellectus est qui facit universalitatem in rebus" (St. Thomas, "De ente et essentia," iv).
Aristotle's great insights into the nature of reality were made possible by his conforming his philosophy to the everyday usage of words. There is great collective wisdom in language.
Modernism is the child of Nominalism, which is why I trace the fall of the West to Ockham's proto-nominalism in the 1300s.
Nominalism, which is irreconcilable with a spiritualistic philosophy and for that very reason with scholasticism as well, presupposes the ideological theory that the abstract concept does not differ essentially from sensation, of which it is only a transformation. The Nominalism of Hume, Stuart Mill, Spencer, Huxley, and Taine is of no greater value than their ideology. They confound essentially distinct logical operations--the simple decomposition of sensible or empirical representations with abstraction properly so called and sensible analogy with the process of universalization. The Aristotleans recognize both of these mental operations, but they distinguish carefully between them.