RELATIONS BETWEEN STIMULUS AND SENSATION 455 



is that a given object, present under the same circumstances, shall always 

 produce the same sign, and that unlike signs shall always correspond to 

 unlike agencies. 



To the popular understanding which assumes the complete truth of the pic- 

 tures represented to us by our senses, this remnant of similarity may appear 

 very meager. But in reality it is not so; for services of the greatest possible 

 moment to us, such as the portrayal of uniformity in the processes of nature, 

 can be performed for us by mere signs. Every natural law declares that condi- 

 tions which are the same in a certain respect are always followed by results 

 which are the same in a certain other respect. Since likeness in our world of 

 sense is signified to us by like signs, the natural sequence of cause and effect 

 will have its counterpart of a perfectly uniform sequence in the realm of sense. 

 If therefore even the qualities of our sensations are nothing but signs which are 

 entirely dependent in kind upon our nervous organization, they are not to be 

 discarded as mere worthless counterfeits. They are signs of something, whether 

 of something merely existing, or something occurring, and what is more im- 

 portant, they are able to portray to us the law of occurrence. 



Physiology, therefore, acknowledges that the nature of sensation is, in the 

 last analysis, subjective. In essence it is transcendental. But since experi- 

 ence proves that excitation of different afferent nerves produces different sen- 

 sations, since we know further that sensation has its correlative physical 

 process not in the excitation of the peripheral sense organ or that of the 

 afferent nerves, but in the activity of the brain, and finally since investigation 

 has shown that different afferent nerves terminate in different fields of the 

 cerebral cortex, which in their turn are connected with other parts of the 

 brain, it follows that the specific character of a sensation is determined by 

 the part of the brain roused to action. It is in this sense that we shall" under- 

 stand the doctrine of specific sensations, as used in this book. 



SECOND SECTION 



THE QUANTITATIVE RELATIONS BETWEEN STIMULUS 



AND SENSATION 



In order that an external stimulus may produce a sensation, it must exceed 

 a certain lower limit of strength, which is called, after Herbart, the threshold 

 value of the stimulus. 1 If the stimulus be increased above this limit, the 

 sensation increases also; but while the strength of the external stimulus may 

 be increased indefinitely the intensity of the sensation never exceeds a certain 

 upper limit. This maximum sensation follows a relatively weak stimulus 

 and a further rise not only does not produce a quantitative increase in the 

 sensation but on the contrary and in ascending degree produces fatigue and 

 exhaustion of the peripheral sense organ. 



1 The threshold value of the stimulus for the different modalities of sensation varies 

 greatly according to circumstances. In absolute terms it may be given as follows : for the 

 sensation of pressure nrWath of an erg; for sensations of sound Trr&^o.oootb of an erg; for 

 sensations of sight (green) wffrr^o.oooth of an erg. 



