PHYSIOLOGICAL PSYCHOLOGY 137
consequence of this independent rule, and not through its own power and impulse, that a change in the soul produces a corresponding one in the body. ... It is quite indifferent to medicine wherein the mys- terious union of soul and body consists, as this is the constant event which lies equally at the bottom of all phenomena. But it is of the greatest interest to medicine to know what affections of the soul are connected in that mysterious manner with what affections of the body." 6 Accordingly, his phenomenal psychology was guided by com- petent knowledge of physics and physiology, the latter, as we must recall, being a subject which he actually professed. His speculative psychology, dealing with the mysterious union, falls within his philosoplry.
The third book of the " Medical Psychology," which still conveys lessons to the physician, deals with subjects such as sleep, attention, emotion, the influence of the flow of consciousness upon secretion, nutrition, and instinct, and with abnormal psychology. The second book reviews the factors of self-consciousness, especially in the light of the relation between the physiological mechanism and the mind. It thus includes his most distinctive contribution to physiological psy- chology — the famous theory of "local signs." This is an integral part of his analysis of space-perception, one of the subtlest ever formu- lated. His latest presentation of it runs thus:
Let it be assumed that the soul once for all lies under the necessity of mentally presenting a certain manifold as in juxtaposition in space; How does it come to localize every individual impression at a definite place in the space intuited by it, in such manner that the entire image thus intuited is similar to the external object which acted on the eye?
Obviously, such a clue must lie in the impressions themselves. The simple quality of the sensation " green " or " red " does not, however, contain it ; for every such color can in turn appear at every point in space, and on this ac- count does not, of itself, require always to be referred to the one definite point.
We now remind ourselves, however, that the carefulness with which the regular position on the retina of the particular excitations is secured, can not be without a purpose. To be sure, an impression is not seen at a definite point on account of its being situated at such a point; but it may perhaps by means of this definite situation act on the soul otherwise than if it were elsewhere situated.
Accordingly we conceive of this in the following way: Every impression cf color " r " — for example, red — produces on all places of the retina, which it reaches, the same sensation of redness. In addition to this, however, it produces on each of these different places, a, b, c, a certain accessory impression, o, /3, 7, which is independent of the nature of the color seen, and dependent merely on the nature of the place excited. This second local impression would therefore be associated with every impression of color " r," in such a manner that ra signifies a red that acts on the point a, r/3 signifies the same red in case it acts on the point b. These associated accessory impressions would, accordingly,
'Ibid., I., pp. 193-97.