There are various reasons, some good, why conceptualism withered. But with the demise of behaviorism, philosophy's linguistic turn growing a bit long in the tooth, and the rise of cognitive science, conceptualism deserves a new hearing. My aim here is to begin building a case that concepts can solve some of the problems of universals, both traditional and recent, and that when they are supplemented by in re properties (ones that exist in the spatio-temporal causal order) they can solve even more. Concepts and in re properties may seem a peculiar twosome, but it bears exploring because it would allow us to avoid abstract entities---creatures that lie outside the spatio-temporal causal order---and the severe epistemological problems they generate. My goal is not to present a definitive version of conceptualism, but to open a space where it can be discussed and evaluated.
In §1 I explain why conceptualism should be of interest now. In §2 I note some central features of concepts and in §3 sketch the major empirical theories designed to explain them. In § 4 I examine ways in which conceptualism could help solve some of the problems of universals.
People did talk about concepts during this period, even referring to their own work as conceptual analysis, but the concepts involved were not much like the mental entities of traditional conceptualists. Indeed, during this period the possession of a concept was often assimilated to the mastery of a word. Concepts (and related mental entities) were also shunned because the Zeitgeist included a general sympathy, sometimes even an enthusiasm, for a behaviorism that had little patience for mentalistic notions.
But times have changed. No one now could seriously maintain that most philosophical problems are at root problems about language, and metaphysics has been respectable for several decades. Moreover, behaviorism is dead, and concepts are now one of the half dozen most central concerns in the flourishing field of cognitive science. It's time to reconsider conceptualism.
If information is to make anything happen in the real world, it must be embodied in something physical, and the dominant view is that it is embodied in mental representations that are realized, somehow, in the brain. Concepts are quintessential mental representations that figure in all the higher mental processes. Even embodied information is useless unless we can do something with it, so we also need mental---computational---operations that process the information encoded in mental representations. Cognitive operations include mechanisms for drawing inferences, retrieving items from memory, and calculating the patterns of movement of our bodies needed to perform the actions we decide to perform.
The key here is that mental operations are both causal mechanisms that make things happen and that the way that they work is sensitive to the content of information encoded in the representations they process. For a simple example suppose that Ann believes that Max is a dog and that all dogs are animals. This may well cause her to infer (validly) that Max is an animal. Nothing guarantees this. Like most dispositions, the activation of information processing dispositions can be thwarted in a variety of ways. Ann may fail to notice the entailment, or the original beliefs may not matter enough to explore their consequences. But if she does conclude that Sam is an animal, it illustrates how causal relations among mental representations can march along in step with their meaning or content.