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. 



