Mechanists have always been known as nominalists. The nominalistic theory of abstract and general terms was the regular mechanistic means of combating the arguments of the formists for the reality of forms and the category of subsistence. Says the traditional mechanist, a form such as blueness or bluejay is nothing but a word which stands for a number of objects. There is no form of bluejay, but there is the word which we have conventionally learned to use in reference to a number of physical objects. Bluejays are grouped into a class simply by virtue of the fact that they are all called by that name.
In this naive formulation nominalism does not carry much conviction. For we are at once prompted to ask how it comes about that this name happens to have been applied to just those physical objects and no others. Is not the reason precisely that those objects do in fact have the common properties of blueness, featheredness, and so on? If we push nominalism no farther, there does not seem to be any adequate answer, and the formists carry the field.
But suppose the mechanist accepts the challenge and asks in terms of his own categories how indeed it does happen that certain names get applied to certain configurations of matter. What, now, is a name? It is a specific response made by an organism on the stimulus of specific environmental configurations. . . . Now a sentence or scientific formula physiologically interpreted is nothing but a combination of such reactions or conditioned reflexes. The whole thing can be causally interpreted. . . . We have hearby developed a causal theory of truth. Nothing is implied about an identity of form between the sentence and the nail. . . . In this mature nominalism what is implied is a system of causal connections which holds between an environmental stimulus and the response of an organism.