SENSATION AND SENSE-ORGANS. 499 



not, therefore, argue as to the nature of the excitant: we 

 have no more warrant for supposing that light- is like our 

 sensation of light than that the knife that cuts us is like our 

 sensation of pain. All that we know with certainty is states 

 of our own consciousness, and although from these we form 

 working hypotheses as to an external universe, yet, granting 

 it, we have no means of acquiring any real knowledge as to 

 the properties of things about us. What we want to know, 

 however, for the practical purposes of life is, not what things 

 are, but how to use them for our advantage, or to prevent 

 them from acting to our disadvantage; and our senses en- 

 able us to do this sufficiently well. 



The Psycho-Physical Law. Although our sensations 

 are, in modality or kind, independent of the force exciting 

 them, they are not so in degree or intensity, at least within 

 certain limits. We cannot measure the amount of a sensa- 

 tion and express it in foot-pounds or calories, but we can get 

 a sort of unit by determining how small a difference in sensa- 

 tion can be perceived. Supposing this smallest perceptible 

 difference to be constant within the range of the same sense 

 (which is not proved), it is found that it is produced by dif- 

 ferent amounts of stimuli, measured objectively as forces; 

 and that there exists in some cases a relation between the two- 

 which can be expressed in numbers. The increase of stimu- 

 lus necessary to produce the smallest perceptible change in a 

 sensation is proportional to the strength of the stimulus 

 already acting; for example, the heavier a pressure already 

 acting on the skin the more must it be increased or dimin- 

 ished in order that the increase or diminution may be felt. 

 Expressed in another way the facts may be put thus: sup- 

 pose three degrees of stimulation to bear to one another ob- 

 jectively the ratios 10, 100, 1000, then their subjective ef- 

 fects, or the amounts of sensation aroused by them, will be 

 respectively as 1, 2, 3: in other words, the sensation increases 

 proportionately to the logarithm of the strength of the stimu- 

 lus. Examples of this, which is known as " Weber's " or 

 " Fechner's psycho-physical law" will be hereafter pointed 

 out, and are readily observable in daily life; we have, for 

 example, a luminous sensation of certain intensity when a 

 lighted candle is brought into a dark room; this sensation is 

 not doubled when a second candle is brought in; and is 

 hardly affected at all by a third. The law is only true, how- 



