Mental imagery (varieties of which are sometimes colloquially refered to as “visualizing,” “seeing in the mind's eye,” “hearing in the head,” “imagining the feel of,” etc.) is quasi-perceptual experience; it resembles perceptual experience, but occurs in the absence of the appropriate external stimuli. It is also generally understood to bear intentionality (i.e., mental images are always images of something or other), and thereby to function as a form of mental representation. Traditionally, visual mental imagery, the most discussed variety, was thought to be caused by the presence of picture-like representations (mental images) in the mind, soul, or brain, but this is no longer universally accepted. Very often, imagery experiences are understood by their subjects as echoes, copies, or reconstructions of actual perceptual experiences from their past; at other times they may seem to anticipate possible, often desired or feared, future experiences. Thus imagery has often been believed to play a very large, even pivotal, role in both memory (Yates, 1966; Paivio, 1986) and motivation (McMahon, 1973). It is also commonly believed to be centrally involved in visuo-spatial reasoning and inventive or creative thought. Indeed, according to a long dominant philosophical tradition, it plays a crucial role in all thought processes, and provides the semantic grounding for language. However, in the 20th century vigorous objections were raised against this tradition, and it was widely repudiated. More recently, it has once again begun to find a few defenders.

Mental imagery is a familiar aspect of most people's everyday experience (Galton, 1880a,b, 1883; Betts, 1909; Doob, 1972; Marks, 1972, 1999). A few people may insist that they rarely, or even never, consciously experience imagery (Galton, 1880a, 1883; Faw, 1997, 2009; but see Brewer & Schommer-Aikins, 2006), but for the vast majority of us, it is a familiar and commonplace feature of our mental lives. The English language supplies quite a range of idiomatic ways of referring to visual mental imagery: ‘visualizing,’ ‘seeing in the mind's eye,’ ‘having a picture in one's head,’ ‘picturing,’ ‘having/seeing a mental image/picture,’ and so on. There seem to be fewer ways to talk about imagery in other sensory modes, but there is little doubt that it occurs, and the experiencing of imagery in any sensory mode is often referred to as ‘imagining’ (the appearance, feel, smell, sound, or flavor of something). Alternatively, the quasi-perceptual nature of an experience may be indicated merely by putting the relevant sensory verb (‘see,’ ‘hear,’ ‘taste,’ etc.) in actual or implied “scare quotes.” Despite the familiarity of the experience, the precise meaning of the expression ‘mental imagery’ is remarkably hard to pin down, and differing understandings of it have often added considerably to the confusion of the already complex and fractious debates, amongst philosophers, psychologists, and cognitive scientists, concerning imagery's nature, its psychological functions (if any), and even its very existence. In the philosophical and scientific literature (and a fortiori in everyday discourse), the expression ‘mental imagery’ (or ‘mental images’) may be used in any or all of at least three different senses, which are only occasionally explicitly distinguished, and all too often conflated:

1. {1} quasi-perceptual conscious experience per se;
2. {2} hypothetical picture-like representations in the mind and/or brain that give rise to {1};
3. {3} hypothetical inner representations of any sort (picture-like or otherwise) that directly give rise to {1}.

Far too many discussions of visual mental imagery fail to draw a clear distinction between the contention that people have quasi-visual experiences and the contention that such experiences are to be explained by the presence of representations, in the mind or brain, that are in some sense picture-like. This picture theory (or pictorial theory) of imagery experience is deeply entrenched in our language and our folk psychology. The very word ‘image,’ after all, suggests a picture. However, although the majority of both laymen and experts probably continue to accept some form of picture theory, many 20th century philosophers and psychologists, from a variety of theoretical traditions, have argued strongly against it, and, in several cases they have developed quite detailed alternative, non-pictorial accounts of the nature and causes of imagery experiences (e.g., Dunlap, 1914; Washburn, 1916; Sartre, 1940; Ryle, 1949; Shorter, 1952; Skinner, 1953, 1974; Dennett, 1969; Sarbin & Juhasz, 1970; Sarbin, 1972; Pylyshyn, 1973, 1978, 1981, 2002a, 2003a, 2005; Neisser, 1976; Hinton, 1979; Slezak, 1991, 1995; Thomas, 1999b, 2009). Others, it should be said, have developed and defended picture theory in sophisticated ways in the attempt to meet these critiques (e.g., Hannay, 1971; Kosslyn, 1980, 1983,1994; von Eckardt, 1988, 1993; Tye, 1988, 1991; Cohen, 1996). However, despite these developments, much philosophical and scientific discussion about imagery and the cognitive functions it may or may not serve continues to be based on the often unspoken (and even unexamined) assumption that, if there is mental imagery at all, it must consist in inner pictures. Consider, for example, the title of the book The Case for Mental Imagery, by Kosslyn, Thompson & Ganis (2006). In fact the book is an extended and quite polemical defense of the much disputed view that visual mental imagery consists in representational brain states that are, in some significant and important ways, genuinely picture-like (see supplement: The Quasi-Pictorial Theory of Imagery, and its Problems). That is to say, the contents suggest that the title should be understood as intending "imagery" in sense {2}. However, it would also be very natural (and, very possibly, in accord with the authors' intentions – compare Kosslyn, Ganis & Thompson, 2003) to understand the title as implying that the book's deeper purpose is to refute the view that imagery, even in sense {1}, does not really exist (or, at least, that the concept of imagery will find no place in a properly scientific ontology). Although this denialist view of imagery has few, if any, supporters today, it is well known that not so very long ago, in the era of Behaviorist psychology, it had great influence. The book's title thus (intentionally or otherwise) invites us to conflate the (now) very controversial view that mental images are picture-like entities, with what is, today, the virtual truism that people really do have quasi-perceptual experiences, and that our science of the mind owes us some account of them. Another way in which the expression ‘mental imagery’ (together with many of its colloquial near-equivalents) may be misleading, is that it tends to suggest only quasi-visual phenomena. Despite the fact that most scholarly discussions of imagery, in the past and today, do indeed focus mainly or exclusively upon the visual mode, in fact, quasi-perceptual experience in other sensory modes is just as real, and, very likely, just as common and just as psychologically important (Newton, 1982). Contemporary cognitive scientists generally recognize this, and interesting studies of auditory imagery, kinaesthetic (or motor) imagery, olfactory imagery, haptic (touch) imagery, and so forth, can be found in the recent scientific literature (e.g., Segal & Fusella, 1971; Reisberg, 1992; Klatzky, Lederman, & Matula, 1991; Jeannerod, 1994; Bensafi et al., 2003; Yoo et al., 2003; Kobayashi et al., 2004; Djordjevic et al., 2004, 2005). Although such studies are still vastly outnumbered by studies of visual imagery, ‘imagery’ has become the generally accepted term amongst cognitive scientists for quasi-perceptual experience in any sense mode (or any combination of sense modes).