Page:Popular Science Monthly Volume 34.djvu/157



HE saying that appearances are deceptive is an inheritance from very ancient times. To Oriental and to Greek philosophers the illusory nature of the knowledge furnished by the senses was a frequent and a fertile theme of contemplation and discussion. The same problem stands open to the psychologist of to-day; but, profiting by the specialization of learning and the advance of technical science, he can give it a more comprehensive as well as a more practical answer. The physiological activities underlying sense-perception are now well understood; the experimental method has extended its domain over the field of mental phenomena; and in every way we have become more expert in addressing our queries to the sphinx, Nature, so as to force a reply. To outline the position of modern psychology with reference to this interesting problem of deception is the object of the present essay.

In a sensation we recognize a primitive element in the acquisition of knowledge. The deprivation of a sense results in a dwarfed and incomplete mental development. This is due, not to the mere sense-impressions that the organ furnishes, but to the perception and co-ordination of these by inferential processes of the higher faculties. It is not the eye of the eagle, but the brain directing the human eye, that gives intellectual supremacy. Physiology recognizes this distinction as one between lower and higher brain-centers. A man may lose his retina, or may have his optic nerve injured, and so be blind in the ordinary sense of the word. He is prevented from acquiring further knowledge by this avenue; but,