On the traces of this brilliant incursion of the natural philosopher into the realm of mental science, later psychologists must follow but haltingly. Just as in the history of physics a long series of studies intervened between Bacon's hypothesis that heat is a kind of motion (1620) and Tyndall's own work, _Heat as a Mode of Motion_ (1863), so must many psychological investigations be made before an adequate psychology of scientific discovery can be formulated. It may ultimately prove that the passages in which Tyndall and other scientists speak of scientific _imagination_ would read as well if for this term, intuition, inspiration, unconscious cerebration, or even reason were substituted.
At first glance it would seem that the study of the sensory elements of consciousness, motor, tactile, visual, auditory, olfactory, gustatory, thermal, internal, pursued for the last half century by the experimental method, would furnish a clue to the nature of the imagination. A visual image, or mental picture, is popularly taken as characteristic of the imaginative process. In fact, the distinguished psychologist William James devotes the whole of his interesting chapter on the imagination to the discussion of different types of imagery. The sensory elements of consciousness are involved, however, in perception, memory, volition, reason, and sentiment, as they are in imagination. They have been recognized as fundamental from antiquity. Nothing is in the intellect which was not previously in the senses. To be out of one's senses is to lack the purposive guidance of the intelligence.
The psychology of individuals and groups shows startling differences in the kind and vividness of imagery. Many cases are on record where the mental life is almost exclusively in visual, in auditory, or in motor terms. One student learns a foreign language by writing out every word and sentence; another is wholly dependent on hearing them spoken; a third can recall the printed page with an almost photographic vividness. The history of literature and art furnishes us with illustrations of remarkable powers of visualization. Blake and Fromentin were able to reproduce in pictures scenes long retained in memory. The latter recognized that his painting was not an exact reproduction of what he had seen, but that it was none the less artistic because of the selective influence that his mind had exerted on the memory image. Wordsworth at times postponed the description of a scene that appealed to his poetic fancy with the express purpose of blurring the outlines, but enhancing the personal factor. Goethe had the power to call up at will the form of a flower, to make it change from one color to another and to unfold before his mind's eye. Professor Dilthey has collected many other records of the hallucinatory clearness of the visual imagery of literary artists.