Traditionally, the mental capacity for experiencing, constructing, or manipulating 'mental imagery' (quasi-perceptual experience). Imagination is also regarded as responsible for fantasy, inventiveness, idiosyncrasy, and creative, original, and insightful thought in general, and, sometimes, for a much wider range of mental activities dealing with the non-actual, such as supposing, pretending, 'seeing as', thinking of possibilities, and even being mistaken. See representation.

Despite being a familiar word of everyday language, imagination is a very complex, contested, and evaluatively loaded concept. It, like many cognate terms, often appears to have radically different senses and connotations when used in different contexts. Furthermore, although it plays only a small overt role in most contemporary theories of the mind, it has played a much more prominent part in the past. The concept will thus best be understood through its history.
Ancient and Medieval Conception
The concept of the imagination seems to have been first introduced into philosophy by Aristotle, who tells us that "imagination [phantasia] is (apart from any metaphorical sense of the word) the process by which we say that an image [phantasma] is presented to us" (De Anima, 428a 1-4). It has been questioned in recent times whether the Greek words phantasia and phantasma are really equivalent to "imagination" and "(mental) image" as heard in contemporary usage. However, there can be little doubt that, at least until very recent times, theoretical discussions of phantasia, its Latin translation imaginatio, and their etymological descendants, continued to be rooted in the concepts introduced by Aristotle and the problems arising from his rather elliptical explanation of them. Very arguably this is true of all Western philosophical schools: Stoics, Epicureans and Neoplatonists quite as much as avowed Aristoteleans; Muslims as much as Christians; and, come to that, Empiricists quite as much as Rationalists.
According to Aristotle "The soul never thinks without a mental image [phantasma]" (De Anima, 431a 15-20). It would appear that, for him (and, again, for most of successors, until very recently), such images played something like the role that is played in contemporary cognitive theory by "mental representations". In this tradition, imagery, and thus imagination, has an essential role to play in all forms of thinking (with the possible exception of direct intuitions of Platonic forms, or of the divine). It has no special connection with inventiveness or creativity.
It does, however, have a special connection with desire. Aristotle argues that our desire for (and, thus, pursuit of) anything not actually present to the senses must be mediated by an image of the desired object. Aristotle's treatment is morally neutral, but his notion of desirous imagination may later have become conflated with the Hebraic concept of yetser, the willful (but also semi-divine) faculty in humanity that led to Adam's (and, indeed, all subsequent) sin. At any rate, in the Judaeo-Christian intellectual tradition (from ancient to relatively recent times) imagination, although recognized as indispensable to cognition, was usually profoundly distrusted. Unless strictly disciplined by reason it would soon lead us into concupiscence and sin.

But, of course, the connection between imagination and perception is the more fundamental. Aristotle's conception of phantasia/imagination seems to be closely bound up with his postulation of what came to be called the "common sense" or sensus communis. This is the part of the psyche responsible for the binding of the deliverances of the individual sense organs into a coherent and intelligible representation, and for apprehending the so called "common sensibles", those aspects of the world (broadly, the spatio-temporal aspects) that can be known through more than one sense mode without being the characteristic proper object of any of them (Aristotelian common sensibles are broadly coextensive with Lockean "primary qualities"). In fact, it is plausible to interpret phantasia and sensus communis as different aspects or modes of a single faculty, depending on whether it is regarded as receptive or productive, or on whether it is operating in the presence or the absence of whatever is being mentally represented. Phantasmata are generated in either case, but when their immediate cause is an object directly before us the tendency is to refer to them as percepts, and to the process as perception; when memory of previously observed things is the source, reference will more likely be to imagery and imagination. Thus imagination came to be particularly associated with thinking about things that are not actually currently present to the senses: things that are not really there.

Some of Aristotle's successors tended to lay the stress on the conceptual separation of the notions of imagination and sensus communis. Thus Early Christian and Medieval anatomists often located sensus communis at the front of the brain's first ventricle, ready to receive sense impressions, whereas imagination was placed at the rear of this ventricle (or even in the second ventricle), and was responsible for holding and perhaps consolidating the resultant images, and passing them back to the other ventricles and faculties (where they could be used in thought or stored in memory). Imagination might also, sometimes, be held responsible for the recombining of various image parts into chimerical forms.
This latter type of process would allow the individual mind a degree of freedom and a scope for idiosyncracy that would hardly have been available from the other traditional faculties, constrained as they were by reality and the laws of logic. It would also, of course, give rise to images even more removed from present actuality than images retrieved intact from memory, and thus even more quintessentially imaginary. In this vein, we sometimes find modern writers making a distinction between "memory imagery" and "imagination imagery", or even restricting the use of "imagination" (and, a fortiori, "imaginary") to thoughts about things that have never (or never yet) been actually experienced. (For some reason, words derived from the original Greek term, such as "fantasy", "fancy", or "phantasm", seem to have come to connote unreality even more strongly than "imagination" and its cognates. It is worth noting that some medieval authors, finding both the Greek and the Latin terms available, did attempt to draw functional and anatomical distinctions between imaginatio and phantasia, but no clear consensus on how this distinction was to be drawn is apparent, and these attempts seem unlikely to have significantly influenced more recent usages.)
Modern Usage
Other ancient and medieval authors, however, were more inclined to stress the underlying identity of the imagination and the common sense, and this interpretation probably became more widespread with the increased availability of more accurate Aristotelian texts and translations in the later middle ages and renaissance. We find this tendency even in such a consciously anti-Aristotelian figure as Descartes, who, in the Treatise on Man, explicitly identifies both the sensus communis and the imagination with the surface of the pineal gland, upon which images (ideés) both of sense and of memory or fancy are inscribed. Although it is important not to confuse these "corporeal ideas" with the "clear and distinct ideas" that play so prominent a part in Descartes' epistemology, it is surely not coincidental that the site of the Cartesian imagination/sensus communis, the pineal surface, is also the privileged site of mind-body interaction, the lynchpin that holds together the two metaphysical worlds of Cartesianism. As it had done for Aristotle, the imagination/sensus communis mediated between the bodily senses, and the (now incorporeal) rational mind.
After the conceptual revolutions of the 17th century the Aristotelian concept of sensus communis largely disappeared from philosophical discussion, and, inasmuch as the functions that had been ascribed to it were considered of interest, they were likely to be directly ascribed to imagination. In the Empiricist tradition, images, under the rubric of "ideas", came more than ever to be seen as the preeminent, even the only, form of mental content. Hume, indeed, frequently uses "imagination" as a virtual synonym for "mind". This is reasonable enough inasmuch as he regards the mind as no more than a bundle of images (impressions and ideas), and cognition as a matter of their vicissitudes and associative interactions. But even for Hume, imagination has a special perceptual role to play. It bears the responsibility (through the way in which it automatically associates similar and contiguous impressions) for our natural tendency to believe in the existence of real and persistent objects existing outside the mind. Likewise (because it also tends to associate similar impressions or impression complexes with certain words) it is responsible for the way that we categorize things into kinds (seeing both Rover and Fido as dogs, for example).
In a sense, for Hume, these latter functions of imagination are defects of the human mind. Philosophical analysis (he argues) shows that our natural belief (engendered by imagination) that the world contains various objects of various kinds cannot be rationally grounded. Kant assigns similar functions to imagination [einbildungskraft], but without such a skeptical twist. Imagination makes knowledge of the phenomenal world possible, by synthesizing the incoherent sensory manifold into representational images suitable to be brought under concepts. The Kantian imagination, in this regard, seems, broadly speaking, to fill much the same role as the Aristotelian sensus communis. However, it has by now become clear just how poorly we understand how any such faculty could possibly work. Kant rejects Hume's purely associative account, and argues that the imaginative synthesis must be governed by a priori rules or schemata, but how they operate is "an art concealed in the depths of the human soul, whose real modes of activity nature is hardly likely ever to allow us to discover" (Critique of Pure Reason A141-B181). The question as to how meaningful mental representations can arise in us remains obscure (or, at the least, very controversial) to this day.
With the rise of the Romantic movement in the latter part of the 18th century, the main focus of discussion concerning imagination shifted away from cognitive theory and epistemology, and towards its role in original, creative thinking, especially in the arts. This role had been acknowledged at least as far back as the work of Flavius Philostratus (c. 3rd century A.D.). Also, unsurprisingly, imagination had a long standing place in discussions of the nature of dreams and visions. What happened with Romanticism was not so much a reconceptualization of imagination, but rather a revaluation of certain concepts that had long been associated with it. Originality, passion, the unreal, and non-rational thought, all generally deprecated in former ages, suddenly achieved a new, strongly positive evaluation. Thus the stock of imagination, as the faculty supposedly responsible for such things, also rose sharply. In this cultural context, the fact that its workings were mysterious became an asset rather than a liability. Hyperbolic praise of the imagination became commonplace in Romantic writings, and certain Romantic aphorisms on the subject became (and remain) intellectual commonplaces. Although some Romantic authors, most notably Samuel Taylor Coleridge, did attempt to develop a new philosophy of imagination adequate to its new high status, in fact they relied heavily on Kant and post-Kantian German idealism (and Plotinus, who himself relied heavily on Aristotle), and the results, although suggestive and much quoted, are (from a philosophical perspective) fragmentary and largely incoherent. Nevertheless, for many intellectuals "imagination" remains a term of great cultural significance, but one whose primary association is with aesthetic theory rather than epistemology, and which is most naturally first approached through the study of the ideas of literary figures such as Coleridge.
Contemporary Usage

Although one major 20th century philosopher (Sartre) wrote two books on the imagination early in his career, by the mid twentieth century the topic had become quite unfashionable in philosophical circles. Most analytical philosophers, where they showed interest at all, seem to have come to doubt whether the imagination even exists. Gilbert Ryle declared, in The Concept of Mind, that "There is no special Faculty of Imagination, occupying itself single-mindedly in fancied viewings and hearings" (1949), and this soon became a widely accepted viewpoint. It was pointed out that although the verb "to imagine" in some contexts seems to be used to refer to the having of imagery, in other contexts this is not obviously the case. For example, it is not immediately apparent that imagining that Goldbach's conjecture has been proven involves (or even can involve) imagery in any central way. Such imagining seems to be more closely akin to supposing, or just believing falsely, than to visualizing. In other contexts, "imagining" seems to be used in a way that is closer to "pretending" or to "thinking of a possibility". How, it was asked, could all of these diverse sorts of mental act be reasonably supposed to be the results of the operation of a single, unitary mental faculty.

It might equally be asked how some trite facts about linguistic usage could be thought to raise a serious challenge to a key component of the cognitive theory that had dominated Western thought almost since its inception. (There are hot dogs, sun dogs, dog ends, and dog day afternoons, and a dogged investigation may involve dogging someone's footsteps. None of them involve canine quadrupeds, but, equally, none of these expressions raise the slightest doubts about the existence of such creatures.) One factor, no doubt, was a reaction against the excesses of Romantic rhetoric, but, more importantly, the traditional imagery centered theories of cognition had come into question for quite different reasons. The combination of the linguistic turn in philosophy and the Behaviorist turn in psychology (itself partly inspired by the "imageless thought" psychology of the Würzburg school) led to a widespread acceptance of the view that thought is ultimately based upon language rather than on imagery, and powerful criticisms of long accepted imagery based theories of linguistic meaning were put forward in the writings of philosophers such as Frege, Wittgenstein, and Moritz Schlick. John B. Watson (1913), the influential instigator of the Behaviorist movement in psychology went so far as to question the very existence of imagery, and although few philosophers went quite this far, the debunking tone taken toward the notion by thinkers as diverse as Ryle and the French author Alain, led to a philosophical climate in which it was generally not taken seriously. Certainly it was no longer ubiquitous in cognitive theory, as it formerly had been, and we should hardly be surprised that in such circumstances it became difficult to see any unifying thread in all the diverse usages of "imagine" that had emerged over the centuries, still less any theoretical need for a faculty of imagination to account for them.

Things changed somewhat in the 1960s early 1970s, when (through the efforts of cognitive psychologists such as Allan Paivio, Roger Shepard, and Stephen Kosslyn) imagery once again became respectable as a topic for experimental psychological investigation. At about the same time, considerations of the need for an explanatory framework capable of handling cognitive process in higher animals and human infants (first language learning, in particular) led theorists away from theories that implied that "natural" (actually spoken) language is representationally basic. However, imagery is still far from regaining acceptance as the fundamental form of mental representation, and current theories of image formation hardly aspire to the central place in cognitive theory once occupied by the imagination. In contemporary Cognitive Science, imagery is usually treated as merely a representationally dependent auxiliary to other, more fundamental and "abstract" forms of mental representation, whether these be the pseudo-languages of "classical" Artificial Intelligence theories (philosophically represented by Jerry Fodor's "mentalese"), the weight spaces and activation patterns of connectionism, attractors in chaotic neural dynamic systems, or whatever.

But if such things do underlie our thought processes, we are certainly not conscious of them as such, and thus their relevance to explaining conscious thought would seem to be, at best, indirect. People are frequently conscious of imagery, however, and it remains very arguable (pace the Würzburg introspectors) that all conscious mental contents are imaginal/perceptual in character. The recently renewed interest in trying to develop a scientific account of consciousness may thus be paving the way for the imagination/sensus communis to be taken seriously once again.
Imagination and Possibility

One other issue deserves mention. According to Hume "'Tis an established maxim in metaphysics, That . . . nothing we imagine is absolutely impossible. We can form the idea of a golden mountain, and from thence conclude that such a mountain may actually exist. We can form no idea of a mountain without a valley, and therefore regard it as impossible." (Treatise, I, ii, 2). (Note that, for Hume, "idea" explicitly meant mental image.) He seems to have meant that whatever is imaginable is possible, and, conversely, if something is impossible it is also unimaginable. Many philosophers have found this view significant and attractive. It would certainly be very convenient for metaphysicians if there were a mental faculty capable of providing a reliable test for possibility!

Hume's maxim is very questionable, however. Although examples that seem to favor it can be multiplied, it is also not hard to come up with apparent counter-examples. It seems to me that I am incapable of imagining curved space-time, but I am reliably informed that it is not only possible but actual. Conversely, countless science fiction buffs have imagined traveling faster than light, which is supposedly impossible. Perhaps some version of the maxim can be saved by sufficiently ingenious maneuvers, probably including the restriction of its scope to some or other subspecies of possibility (perhaps it applies to logical, conceptual, or metaphysical, but not to physical possibility), but such issues lie well beyond the scope of this discussion. It is, however, worth mentioning that the maxim has very little purchase if imagination is interpreted after the fashion of those who would deny its essential connection with imagery. Clearly we can pretend or mistakenly believe that impossible things are possible, and we suppose an impossibility every time we set up a sound reductio ad absurdum proof. On the other hand, if "imagination" is to be understood (as some recent philosophers claim) as something like "the faculty that envisages possibilities" (Rorty, 1988; c.f. White, 1990), then Hume's maxim would seem to be true but trivial. Who but a philosopher, however, would dream of denying that imagination has to do with imagery?
Nigel J.T. Thomas