2O PSYCHOLOGY AND PREACHING
tion brings us up squarely against a stone wall beyond which we cannot go. It plunges us into the old problem which has been the philosophical puzzle of the ages. Really we can only compare states of consciousness with one an other. All that we can or need say here is that the mental image is constituted in experience. It is the resultant of the reaction of the conscious organism to a stimulus perhaps is that reaction itself and by it the organism is enabled to recognize the same stimulus when it recurs. In the ex perience some modification of the brain substance seems to occur, though it is quite difficult to conceive of the exact nature of this modification. However, it seems clear that the modification of the cellular structure of the brain can not be said to be the image, because the latter is a phase of consciousness, and the former is supposed to continue to exist during a lapse of consciousness ; but it is the physical basis, or counterpart, or coefficient of the psychic fact. In brief, then, we may define a mental image as a conscious copy of an experience. Further than this we cannot go in the inquiry as to the nature of the image without passing out of the territory of psychology proper into that of the theory of knowledge.
I. Forms of Imagery. There is a form of imagery cor responding to each of the modes of sensation visual, auditory tactual, gustatory, olfactory, kinesthetic, etc. A perfectly normal person would be able to form mental images corresponding to all these forms of experience ; and, there fore, the inner world of images should be a psychic counter part of the whole environment as experienced in sensation. But the perfectly normal mind is probably not in existence. As a matter of fact persons differ greatly in their capac ity for the several forms of imagery. Some have little capacity, or but a rudimentary one, for visual images, while having a strong faculty for auditory or other forms; or vice versa. Again, those who are endowed with an excel lent capacity for visual images may be able to see with the eye of the mind only still objects, while others can readily
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