Machery (Reference Machery2009) provides a substantial and convincing argument that prototypes, exemplars, and theories form distinct mental kinds. The real question is whether or not this heterogeneity should cause us to eliminate concepts as a mental kind. Neo-empiricism (e.g., Barsalou Reference Barsalou1999; Glenberg Reference Glenberg1997; Prinz Reference Prinz2002) poses a direct threat to this proposal because it provides a unified account of concepts. As Machery notes, neo-empiricism involves two core claims: that conceptual knowledge is encoded in sensorimotor representations and that conceptual processing involves some kind of perceptual simulation. If these claims apply to prototypes, exemplars, and theories, then Machery's case for eliminativism falls apart. This is not simply a hypothetical argument; at least one prominent neo-empiricist (Prinz Reference Prinz2002) endorses both the independence of prototypes, exemplars, and theories and the theoretical unity of concepts.

In Doing without Concepts, Machery claims that neo-empiricism faces two main problems. The first is that certain amodal models (i.e., ones containing non-perceptual representations) are compatible with the empirical findings cited in support of neo-empiricism. The second is that many amodal theorists acknowledge that perceptual imagery is important to some cognitive processes. Machery (Reference Machery2009, p. 116) concludes that “there is no strong evidence that concepts (or some concepts) are in fact similar to perceptual representations.” This argument is inconclusive, though, because it fails to provide strong evidence against perceptually based conceptual representations or, indeed, for amodal ones.

In his Précis, Machery identifies an additional problem for neo-empiricism, which he refers to as the generality problem. This problem arises because there are robust bodies of evidence suggesting that some specialized conceptual representations are amodal. Although the generality problem clearly undermines strong forms of neo-empiricism, it is compatible with weaker forms (Machery Reference Machery2007; Weiskopf Reference Weiskopf2007).

I suggest that neo-empiricism faces a more serious problem. This problem is not merely a lack of generality but rather the presence of heterogeneity. Evidence from a number of sources suggests that conceptual knowledge is encoded in multiple representational formats, some of which are not perceptual (Dove Reference Dove2009). In other words, it supports what might be called the representational heterogeneity hypothesis (RHH).

The case for the RHH can be seen most clearly in recent neuropsychological and neuroimaging research on imageability. Traditionally, cognitive scientists examined imageability in terms of processing advantages for high-imageable concepts over low-imageable ones in several cognitive tasks (Paivio Reference Paivio1987; Wattenmaker & Shoben Reference Wattenmaker and Shoben1987). Two major theories emerged in response to the behavioral evidence: the dual-code theory (Paivio Reference Paivio1987) and the context-availability theory (Schwanenflugel & Shoben Reference Schwanenflugel and Shoben1983). Recent evidence from cognitive neuroscience supports both to some degree (Dove Reference Dove2009).

Because my concern is the RHH, I will focus on the evidence for distinct representational formats. First, consider neuropsychological case studies. Several research teams describe aphasic patients with significant left hemisphere damage who exhibit a selective semantic impairment for high-imageable words (Berndt et al. Reference Berndt, Haendiges, Burton and Mitchum2002; Bird et al. Reference Bird, Howard and Franklin2003; Crepaldi et al. Reference Crepaldi, Aggujaro, Arduino, Zonca, Ghirardi, Inzaghi, Colombo, Chierchia and Luzzatti2006). Patients with a selective semantic impairment for low-imageable words have also been found (Marshall et al. Reference Marshall, Pring, Chiat and Robson1996).

Second, a number of event-related potential (ERP) experiments support a neuroanatomical distinction between concepts of high and low imageability. For example, Holcomb et al. (Reference Holcomb, Kounios, Anderson and West1999) created a task that involved manipulations of both context and concreteness. ERP recordings were time-locked to sentence final words in a word-by-word reading task in which participants made semantic congruency judgments (e.g., Armed robbery implies that the thief used a weapon vs. Armed robbery implies that the thief used a rose). They found that sentence-final concrete words generated a larger and more anterior N400 than sentence-final abstract words in both contexts. Further studies have found context-independent topographic effects associated with imageability in single-word presentations (Kellenbach et al. Reference Kellenbach, Wijers, Hovis, Mulder and Mulder2002; Swaab et al. Reference Swaab, Baynes and Knight2002). Thus, ERP studies employing diverse tasks support the notion that different cognitive systems are associated with the semantic processing of high- and low-imageable words.

Third, the idea that neural activity is modulated by imageability is generally supported by neuroimaging data. A number of studies find that high-imageable words elicit greater activation than low-imageable words in superior regions of the left temporal lobe (Binder et al. Reference Binder, Westbury, McKiernan, Possing and Medler2005; Giesbrecht et al. Reference Giesbrecht, Gamblin and Swaab2004; Mellet et al. Reference Mellet, Tzourio, Denis and Mazoyer1998; Noppeney & Price Reference Noppeney and Price2004; Sabsevitz et al. Reference Sabsevitz, Medler, Seidenberg and Binder2005) and inferior regions of the left prefrontal cortex (Binder et al. Reference Binder, Westbury, McKiernan, Possing and Medler2005; Giesbrecht et al. Reference Giesbrecht, Gamblin and Swaab2004; Goldberg et al. Reference Goldberg, Perfetti and Schneider2006; Noppeney & Price Reference Noppeney and Price2004; Sabsevitz et al. Reference Sabsevitz, Medler, Seidenberg and Binder2005). Giesbrecht et al. (Reference Giesbrecht, Gamblin and Swaab2004), for example, manipulated both imageability and semantic priming (a measure of the influence of context) in an event-related fMRI study. Participants were presented with prime word followed by a target word. The words were either semantically related (bread and butter) or unrelated (wheat and slipper). Half of the pairs consisted of two high-imageable words, and half of the pairs consisted of two low-imageable words. In keeping with the general finding that context effects are distinct from imageability effects, each of these manipulations modulated activity in anatomically distinct areas of the left hemisphere.

The RHH seems to be grist for Machery's mill. It certainly undermines the threat posed by neo-empiricism to unify concepts. In keeping with this, Piccinini and Scott (Reference Piccinini and Scott2006) have argued that the divide between cognitive processes that require language and those that do not provides a better case for conceptual eliminativism than the heterogeneity identified by Machery.

However, the RHH also raises an interesting problem. Because there is every reason to suppose that prototypes, exemplars, and theories have high- and low-imageable variants, these kinds are likely to be representationally heterogeneous. Following Machery's reasoning, this should threaten their status as theoretical kinds. It seems reasonable to resist this inference, but then the challenge is to provide an argument for resisting eliminativism in the context of prototypes, exemplars, and theories that does not apply to the larger context of concepts. Whether this can be accomplished or not remains to be seen.