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Various defenses of amodal symbol systems are addressed, including amodal symbols in sensory-motor areas, the casual theory of concepts, supramodal concepts, latent semantic analysis, and abstracted amodal symbols. Various aspects of perceptual symbol systems are clarified and developed, including perception, features, simulators, category structure, frames, analogy, introspection, situated action, and development. Particular attention is given to abstract concepts, language, and computational mechanisms.

Prior to the twentieth century, theories of knowledge were inherently perceptual. Since then, developments in logic, statistics, and programming languages have inspired amodal theories that rest on principles fundamentally different from those underlying perception. In addition, perceptual approaches have become widely viewed as untenable because they are assumed to implement recording systems, not conceptual systems. A perceptual theory of knowledge is developed here in the context of current cognitive science and neuroscience. During perceptual experience, association areas in the brain capture bottom-up patterns of activation in sensory-motor areas. Later, in a top-down manner, association areas partially reactivate sensory-motor areas to implement perceptual symbols. The storage and reactivation of perceptual symbols operates at the level of perceptual components - not at the level of holistic perceptual experiences. Through the use of selective attention, schematic representations of perceptual components are extracted from experience and stored in memory (e.g., individual memories of green, purr, hot). As memories of the same component become organized around a common frame, they implement a simulator that produces limitless simulations of the component (e.g., simulators of purr). Not only do such simulators develop for aspects of sensory experience, they also develop for aspects of proprioception (e.g., lift, run) and introspection (e.g., compare, memory, happy, hungry). Once established, these simulators implement a basic conceptual system that represents types, supports categorization, and produces categorical inferences. These simulators further support productivity, propositions, and abstract concepts, thereby implementing a fully functional conceptual system. Productivity results from integrating simulators combinatorially and recursively to produce complex simulations. Propositions result from binding simulators to perceived individuals to represent type-token relations. Abstract concepts are grounded in complex simulations of combined physical and introspective events. Thus, a perceptual theory of knowledge can implement a fully functional conceptual system while avoiding problems associated with amodal systems. Implications for cognition, neuroscience, evolution, development, and artificial intelligence are explored. Keywords: analogue processing; categories; concepts; frames; imagery; images; knowledge; perception; representation; sensory-motor representations; simulation; symbol grounding; symbol systems

Something of the relation of my work on substance concepts to Gibsonian theories of perception-action is discussed. What historical relations tie a particular substance concept to a particular substance is discussed.

Millikan (1998a) relies on the ability an organism may have to reidentify external objects. It is difficult to develop an account of how this might occur because the organism could make a mistake in the tokening of a concept; it could misidentify the external object. To sustain her non-descriptivism, Millikan's account of reidentification must make the link between concept and object arbitrary. However, to make mistakes possible, there must be a norm for the production of concepts. These two requirements seem to leave no room for a middle ground.

The meanings of abstract concepts depend on context. Perceptual symbol systems (PSS) provide a powerful framework for representing such context. Whereas a few expected difficulties for simulations are consistent with empirical findings, the theory does not clearly predict simulations of specific abstract concepts in a testable way and does not appear to distinguish abstract noun concepts (like truth) from their stem concepts (such as true).

I criticize three claims concerning simulators: (1) That a simulator provides the best-fitting simulation of the perceptual impression one has of an object does not guarantee, pace Barsalou, that the object belongs to the simulator's category. (2) The people described by Barsalou do not acquire a concept of truth because they are not sensitive about the potential inadequacy of their sense impressions. (3) Simulator update prevents Barsalou's way of individuating concepts (i.e., identifying them with simulators) from solving the problem of intra-personal concept stability because to update a simulator is to change its content, and concepts with different contents are distinct.

Barsalou's hypothesis that mental representations are constructed by selecting parts of percepts encounters the same difficulties as other empiricist theories: They cannot explain concepts for which instances do not share perceptible features (e.g., furniture) or for which there are no relevant percepts (e.g., the end of time). Barsalou's attempt to reduce falsity to failed pattern matching is an elementary error, and the generativity of his simulators cannot be attained without nonterminal symbols. There is not now, and there never was, any reason to be interested in empiricist theories of knowledge. Abstraction is a fundamental aspect of human cognition.

The perceptual symbol system view assumes that perceptual representations have a role-argument structure. A role-argument structure is often incorporated into amodal symbol systems in order to explain conceptual functions like abstraction and rule use. The power of perceptual symbol systems to support conceptual functions is likewise rooted in its use of structure. On Barsalou's account, this capacity to use structure (in the form of frames) must be innate.

Barsalou is right in arguing that perception has been unduly neglected in theories of concept formation. However, the theory he proposes is a weaker version of the classical empirical hypothesis about the relationship between sensation, perception, and concepts. It is weaker because it provides no principled basis for choosing the elementary components of perception. Furthermore, the proposed mechanism of concept formation, growth and development - simulation - is essentially equivalent to the notion of a concept, frame, or theory, and therefore inherits all the well-known problems inherent to these constructs. The theory of simulation does not provide a clearly better alternative to existing notions.

In their quality as acoustic or visual percepts, words are linked to the emotional values of the state-of-affairs they evoke. This allows them to engender meanings capable of operating nearly entirely detached from percepts. Such a laying flat of meanings permits deliberation to take place within the window of consciousness. In such a theatre of the imagination, linguistically triggered, resides the originality of the human psyche.

We agree with Barsalou's claim about the importance of perceptual symbols in a theory of abstract concepts. Yet we maintain that the richness of many abstract concepts arises from the metaphorical mapping of recurring patterns of perceptual, embodied experience to provide essential structure to these abstract ideas.

Three major attempts by Barsalou to specify what makes a perceptual symbol perceptual fail. One way to give such an account is to use symbols' causal/nomic relation to what they represent, roughly the way contemporary informational psychosemanticists develop their theories, but this fails to draw the distinction Barsalou seems to have in mind.

The problems that Millikan addresses in theories of concepts arise from an extensional view of concepts and word meaning. If instead one assumes that concepts are psychological entities intended to explain human behavior and thought, many of these problems dissolve.

Millikan's proposal of a common structure of substance concepts does not explain certain conspicuous findings in the psychological literature such as typicality effects, the context sensitivity of these effects, anti slips of the tongue. Moreover, it is unclear how antidescriptionism could be relevant to psychological theorizing. Finally, it does not seem to be true that concepts of individuals, stuff, and real kinds have a common structure in older children and in adults.

The computational notion of 'trading spaces' is highly relevant to the psychological domain of categorization. The 'theory' view of concepts can be interpreted as a recoding view. A design principle for exploiting learned recodings in order to handle the type-2 problem of forming sophisticated concepts is outlined.

Glenberg offers two different accounts of embodied conceptualization. The first fails in cases where no direct bodily interaction is possible. The second fails in cases where the object in question cannot serve as a discriminative stimulus; moreover, it yields inappropriate content even in cases where it can be applied. Glenberg's disregard for the conceptual agenda set by the social world is also disquieting.

If Knobe is right that ordinary judgments are normatively suffused, how do scientists free themselves from these influences? I suggest that because science is distributed and externalized, its claims can be manipulated in ways that allow normative influences to be hived off. This allows scientists to deploy concepts which are not normatively suffused. I suggest that there are good reasons to identify these normatively neutral concepts with the folk concepts. doi: 10.1017/S0140525X10001809

Understanding the role of simulation in conceptualization has become a priority for cognitive science. Barsalou makes a valuable contribution in that direction. The present commentary points to theoretical issues that need to be refined and elaborated in order to account for key aspects of meaning construction, such as negation, counterfactuals, quantification or analogy. Backstage cognition, with its elaborate bindings, blendings, and mappings, is more complex than Barsalou's discussion might suggest. Language does not directly carry meaning, but rather serves, along with countless other situational elements, as a powerful instrument for prompting its construction.

Our concerns fall into three areas: (1) Barsalou fails to make clear what simulators are (vs. what they do); (2) activation of perceptual areas of the brain during thought does not distinguish between the activation's being constitutive of concepts or a mere causal consequence (Barsalou needs the former); and (3) Barsalou's attempt to explain how modal symbols handle abstraction fails.