An Introduction to the Science and Philosophy of Mental Imagery

Abstract

Pylyshyn's critique is powerful. Pictorial theories of imagery fail. On the other hand, the symbolic description theory he manifestly still favors also fails, lacking the semantic foundation necessary to ground imagery's intentionality and consciousness. But, contrary to popular belief, these two theory types do not exhaust available options. Recent work on embodied, active perception supports the alternative perceptual activity theory of imagery

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Pylyshyn's return to the fray of the imagery debate is very welcome. In typically trenchant fashion he sets forth the serious conceptual and empirical problems afflicting pictorial (including "quasi-pictorial") theories of imagery, showing how even vaunted neuro-imaging evidence fails to support it. Despite its surface appeal, pictorialism is almost certainly false.

However, much as I value Pylyshyn's new contribution, I fear his re-entry into the debate may serve to further entrench a false dichotomy that seems firmly established in the minds of most cognitive scientists, and in the textbooks: the view that we are faced with a stark choice between some form of pictorialist theory of imagery1, or, alternatively, a "propositional" theory wherein imagery (quasi-perceptual experiences and associated empirical effects) is identified with descriptions couched in a computational language of thought (Fodor's (1975) mentalese). Pylyshyn is reticent about his positive theory of imagery (indeed, I think it has never been expounded in detail2), but clearly, under the guise of "the null hypothesis," he wants to sell us the same "propositional" descriptionist theory long associated with his name.

Because its details remain so underspecified, and because mentalese is supposed, ex hypothesis, to be able to represent anything that we can conceive, there are very few empirical constraints on descriptionism as it stands. Virtually any conceivable empirical observation could be accommodated without too much strain. Probably largely because of this lack of empirical content, descriptionism has remained unpopular, despite all the problems of pictorialism.

There is a worse problem, however. Mentalese is conceived by analogy to natural languages such as English and to computer programming languages and representation systems set up within actual working computer programs. Inasmuch as the symbols of such systems represent anything in the outside world, they do so by convention or by stipulation, which requires beings with minds to be around to do the stipulating or to settle upon the conventions. The whole point of mentalese, however, is to explain how minds are possible. Fodor postulated it largely to explain the constitutive intentionality of thought, the fact that thoughts, including mental images, are semantically meaningful, are thoughts or images of something or other. Thus, to explain mentalese semantics as stipulative or conventional would be viciously circular.

Admittedly, a lot of philosophical effort, over the last quarter century, has gone into trying to devise a naturalistic semantics for mentalese, one not dependent upon stipulation or convention. But this work has not even begun to converge upon any generally acceptable theory. The literature has become a casuistical morass, where every positive proposal (the underlying idea often, now, obscured beneath a mass of accumulated modifications) seems decisively refuted, even thoroughly incoherent, from the perspective of its rivals3. If a naturally meaningful mentalese really could exist, it would explain an awful lot, but then again, so would a homunculus. It is past due time to admit that the quest for such a language is hopeless. Certainly we cannot take the conceptual legitimacy of mentalese for granted.

This need not threaten computational theories of specific cognitive competencies and performances, which rarely need to invoke intentionality. Once we drop the requirement that computational cognitive representations should bear or ground intentionality, less problematic accounts of them become available (e.g. Cummins, 1996; Horst, 1996). However, the descriptionist is not just explaining competencies and performances, he is trying to explain imagery, a quintessentially intentional and conscious phenomenon (Sartre, 1948; Thomas, in press). (Some philosophers hold that consciousness may be explicable in terms of representations, but these proposals rely upon the representations bearing intentionality (Lycan, 2000).)

Picture theorists should not cheer, however. The only plausible account of the intentionality of mental pictures or quasi-pictures is that it derives from the intentionality of mentalese (Fodor, 1975; Tye, 1991; Thomas, in press §3.2). Without mentalese (or, worse, a homuncular mind's-eye) inner pictures will be neither intentional nor consciously experienced.

But the assumption that pictorialism and descriptionism exhaust our options for explaining imagery arises from mere historical accident. In the 1970s computational cognitive science was a new, exciting paradigm, but imagery, prima facie a thoroughly un-computational phenomenon, conscious and informal, was also a newly fashionable topic in psychology, with experimental evidence emerging demonstrating its objective reality and functional significance (Kessel, 1972; Thomas, in press §2.1). The notorious "imagery debate" of that era was really about how and whether the evidence on imagery could be reconciled with symbolic computationalism, and Kosslyn's quasi-pictorialism (1980) soon emerged to rival Pylyshyn's descriptionist answer. Around the same time, several psychologists, (sensitive, like Pylyshyn, to the defects of pictorialism) suggested alternative, non-computational mechanisms for imagery, versions of what I call perceptual activity theory4, but their voices were drowned by the clamor of the computationalists' urgent debate.

Circumstances today are very different. Symbolic computationalism has lost much of its luster, and is certainly is no longer "the only game in town". With the emergence of embodied and situated approaches to cognition (not to mention connectionism and dynamical systems theory) we need no longer remain locked into a dichotomous choice of theories developed to appease symbolic computationalists. Perceptual activity theory coheres well with these newer approaches to cognition and has distinct conceptual and empirical advantages over both quasi-pictorialism and descriptionism (Thomas, 1999). It also suggests a promising approach to naturalizing intentionality and consciousness (Thomas, 1999, 2001).

Pylyshyn's critique appeals to O'Regan's work, but O'Regan's conclusions (1992; O'Regan & Noë, 2001) are incompatible with pictorialism and descriptionism alike. Visual experience, he holds, arises not from the presence of representations in the brain but from the active exercise of our "mastery of the relevant sensorimotor contingencies" (O'Regan & Noë, 2001) as we explore our visual surroundings. Perceptual activity theory holds that imagery arises from vicarious exercise of such mastery: a sort of play-acting of perceptual exploration (Thomas, 1999). Although the evidence does not support pictorialism we should not thereby conclude that Pylyshyn's "null hypothesis" is true, or even null.