That humans can categorize in different ways does not imply that there are qualitatively distinct underlying natural kinds or that the field of concepts splinters. Rather, it implies that the unitary goal of forming concepts is important enough that it receives redundant expression in cognition. Categorization science focuses on commonalities involved in concept learning. Eliminating “concept” makes this more difficult.

Machery's Heterogeneity Hypothesis depends on his argument that no theory of concepts can account for all the extant reliable categorization data. I argue that a single theoretical framework based on graphical models can explain all of the behavioral data to which this argument refers. These different theories of concepts thus (arguably) correspond to different special cases, not different kinds.

In this commentary, I make three points concerning Machery's response to neo-empiricism. First, his methodological critique fails to remove the threat that neo-empiricism poses to his conceptual eliminativism. Second, evidence suggests that there are multiple semantic codes, some of which are not perceptually based. Third, this representational heterogeneity thwarts neo-empiricism but also raises questions with respect to how we should “do without concepts.”

This commentary raises two concerns with Machery's approach in Doing without Concepts. The first concern is that it may be possible to preserve a unified theory of concepts by distinguishing facts about concept individuation from facts about cognitive structures and processes. The second concern questions the sharpness of the distinction Machery draws between psychological and philosophical conceptions of concepts.

Machery's case against hybrids rests on a principle that is too strong, even by his own lights. And there are likely important generalizations to be made about hybrids, if they do exist. Moreover, even if there were no important generalizations about concepts themselves, the term picks out an important class of entities and should be retained to help guide inquiry.

Distinct systems for representing concepts as prototypes, exemplars, and theories are closely integrated in the mind, and the notion of concept is required as a framework for exploring this integration. Eliminating the term “concept” from our theories will hinder rather than promote scientific progress.

Machery suggests that the concept of “concept” is too heterogeneous to serve as a “natural kind” for scientific explanation, so cognitive science should do without concepts. I second the suggestion and propose substituting, in place of concepts, inborn and acquired sensorimotor category-detectors and category-names combined into propositions that define and describe further categories.

We critically review key lines of evidence and theoretical argument relevant to Machery's “heterogeneity hypothesis.” These include interactions between different kinds of concept representations, unified approaches to explaining contextual effects on concept retrieval, and a critique of empirical dissociations as evidence for concept heterogeneity. We suggest there are good grounds for retaining the concept construct in human cognition.

Some things in our environment are not what they seem, and they provide a challenge to theories of concepts that emphasize similarity. Section 1 of my commentary explores a dilemma this situation creates for Machery. Section 2 describes a more general problem for prototype and exemplar theories. Section 3 locates a place for similarity-based concepts, and indicates an alternative to Machery's thesis.

Machery rightly points out a diverse set of phenomena associated with concepts that create challenges for many traditional views of their nature. It may be premature, however, to give up such views completely. Here I defend the possibility of hybrid models of concept structure.

Machery has usefully organized the vast heterogeneity in conceptual representation. However, we believe his argument is too narrow in tacitly assuming that concepts are comprised of only prototypes, exemplars, and theories, and also that its eliminative aspect is too strong. We examine two exceptions to Machery's representational taxonomy before considering whether doing without concepts is a good idea.

This commentary focuses on Machery's eliminativist claim, that “concept” ought to be eliminated from the theoretical vocabulary of psychology because it fails to denote a natural kind. I argue for the more traditional view that concepts are a functional kind, which provides the simplest account of the empirical evidence discussed by Machery.

Machery emphasizes the centrality of explanation for theory-based approaches to concepts. I endorse Machery's emphasis on explanation and consider recent advances in psychology that point to the “heterogeneity” of explanation, with consequences for Machery's heterogeneity hypothesis about concepts.

Concepts are mental symbols that have semantic structure and processing structure. This approach (1) allows for different disciplines to converge on a common subject matter; (2) it promotes theoretical unification; and (3) it accommodates the varied processes that preoccupy Machery. It also avoids problems that go with his eliminativism, including the explanation of how fundamentally different types of concepts can be co-referential.

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.