FECHNER'S LAW. 473 



ism; they depend on properties of our Bodies and not on 

 properties of external things, except in so far as these may 

 or may not be adapted to arouse our different sensory 

 apparatuses to activity. From the kind of the sensation we 

 cannot, 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 knowl- 

 edge 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, bat how to use them for our advan- 

 tage, or to prevent them from acting to our disadvantage; 

 and our senses enable 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 cer- 

 tain limits. We cannot measure the amount of a sensation 

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

 sort of unit by determining how small a difference iii sen- 

 sation can be perceived. Supposing this smallest perceptible 

 diffenmce 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. 'jTJie increase of stim- 

 ulus 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 iW 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 in- 



