PROFESSOR AT HEIDELBERG 237 



theory, that spatial separation is, generally speaking, to be 

 predicated only of such sensations as can be separated by 

 actual movement from each other '. We learn to interpret the 

 signs, by comparing them with the results of our movements 

 and with the changes we can produce by means of the latter in 

 the external world. According to Helmholtz the only difference 

 between the inferences of the logicians and those of induction 

 (the results of which become evident in the percepts of the 

 external world as derived from experience) is, that the former 

 can be expressed in words, while in the latter words are 

 replaced by memory images of sensations. This region of 

 the conceptional faculty combines only those sensory im- 

 pressions which are not capable of expression in words ; ' in 

 Germany we term this Cognition (das Kennen)' 



The 1868 lectures on ' Recent Progress in the Theory of 

 Vision ' were an amplification of certain points in Physiological 

 Optics, in which Helmholtz with his accustomed brilliancy and 

 perspicacity gathered up some details of general interest, which 

 would have been overlooked in the larger work. 



In describing the defects of the optical apparatus of the eye, 

 he insists (in conformity with his empiricist attitude) ' that it is 

 not the mechanical perfection of the sensory instrument that 

 creates these marvellously true and exact impressions', and 

 after discussing visual sensation, and the theory of colour, after- 

 images, and contrast, he says : ' Whatever inexactness and 

 incompleteness we may have found in the optical apparatus 

 and retinal image, is as nothing compared with the incon- 

 gruences which we encounter in the region of sensation. We 

 are tempted to believe that Nature had advisedly perpetrated 

 the wildest contradictions, and was determined to destroy all 

 dreams of a pre-established harmony between the outer and 

 inner world.' 



In his Commemorative Lecture on Helmholtz, du Bois- 

 Reymond remarks that 'just as the principle of the conservation 

 of energy has been a safe clue to Helmholtz's train of thought 

 in the preceding period, so in the later part we have a similar 

 guide. The fundamental principle of these researches is the 

 empiricist attitude, which Helmholtz favours in preference to 

 the nativistic, which he rejected. This is the same contrast 

 that obtained in the sixteenth century between Leibniz's pre- 



