Machery argues that concepts are too heterogeneous to be a natural kind. I argue that the book does not go far enough. Theories of concepts assume that the task of categorizing warrants a unique set of cognitive constructs. Instead, cognitive science must look across tasks to find a fundamental set of cognitive mechanisms.

Machery neglects the crucial role of concepts in psychological explanation, as well as the efforts of numerous “externalists” of the last 40 years to provide an account of that role. He rightly calls attention to the wide variation in people's epistemic relations to concepts – people's conceptions of things – but fails to appreciate how externalist and kindred proposals offer the needed stability in concepts themselves that underlies that variation.

Machery argues that concepts do not constitute a natural kind. We argue that this is a mistake. When appropriately construed, his discussion in fact bolsters the claim that concepts are a natural kind.

I argue that Machery stacks the deck against hybrid theories of concepts by relying on an unduly restrictive understanding of coordination between concept parts. Once a less restrictive notion of coordination is introduced, the empirical case for hybrid theories of concepts becomes stronger, and the appeal of concept eliminativism weaker.

Focusing on Machery's claim that concepts play entirely different roles in philosophy and psychology, I explain how one well-known philosophical theory of concepts, Conceptual Atomism (CA), when properly understood, takes into account both kinds of roles.

The first seven chapters of Doing without Concepts offer a perfectly reasonable view of current research on concepts. The last chapter, on which the central thesis of the book rests, provides little actual evidence that using the term “concept” impedes scientific progress. It thus fails to demonstrate that this term should be eliminated from the scientific vernacular.

We argue that Machery provides no convincing evidence that prototypes and exemplars are typically used in distinct cognitive processes. This partially undermines the fourth tenet of the Heterogeneity Hypothesis and thus casts doubts on Machery's way of splitting concepts into different kinds. Although Machery may be right that concepts split into different kinds, such kinds may be different from those countenanced by the Heterogeneity Hypothesis.

Machery proposes that the construct of “concept” detracts from research progress. However, ignoring development also detracts from research progress. Developmental research has advanced our understanding of how concepts are acquired and thus is essential to a complete theory. We propose a framework that both accounts for development and holds great promise as a new direction for thinking about concepts.

Machery denies the traditional view that concepts are constituents of thoughts, and he more provocatively argues that concepts should be eliminated from our best psychological taxonomy. I argue that the constituency view has much to recommend it (and is presupposed by much of his own theory), and that the evidence gives us grounds for pluralism, rather than eliminativism, about concepts.

We evaluate the heterogeneity hypothesis by considering the developmental time course and the mechanism of acquisition of exemplars, prototypes, and theories. We argue that behavioral and modeling data point to a sequential emergence of these three types of concepts within a single system. This suggests that similar or identical underlying cognitive processes – rather than separate ones – underpin representation acquisition.

Machery's dismissive position on parsimony requires that we examine especially carefully the data he provides as evidence for his complex triple-system account. We use the prototype-exemplar debate as an example of empirical findings which may not, in fact, support a multiple-systems account. We discuss the importance of considering complexity in scientific theory.

In this response, I begin by defending and clarifying the notion of concept proposed in Doing without Concepts (Machery 2009) against the alternatives proposed by several commentators. I then discuss whether psychologists and philosophers who theorize about concepts are talking about distinct phenomena or about different aspects of the same phenomenon, as argued in some commentaries. Next, I criticize the idea that the cognitive-scientific findings about induction, categorization, concept combination, and so on, could be explained by positing a single kind of concept, and I insist that many categories (substances, types of events, etc.) are represented by distinct coreferential concepts that belong to very different kinds of concept. This is followed by an assessment of the hybrid theories of concepts offered by commentators, according to which categories, substances, and types of events are represented by hybrid concepts made of several parts. Finally, I defend the proposal that it may be useful to eliminate concept from the theoretical vocabulary of psychology.