i 4 o HERMANN VON HELMHOLTZ 



venerated Kant or Hegel. He had long been convinced that 

 if the physicist tests the galvanometer and telescope he intends 

 to work with to the limits of their efficacy, it is no less in- 

 cumbent on a scientific man to include the intellect in the 

 sphere of his investigations, in order to ascertain what he can 

 arrive at by its means, and where it is likely to fail him. Helm- 

 holtz was fully aware that he had on the one hand 'all the 

 metaphysicians, including the materialists, and all minds with 

 lurking metaphysical tendencies ', to reckon with, while on the 

 other, the scientific world would be impelled by the excrescences 

 of Hegel's ' nature-philosophy ' to extreme suspicion of all 

 speculative explanations of natural phenomena, and would 

 extend this legitimate prejudice to the epistemological and 

 psychological investigations in which the attempt to penetrate 

 the laws of mental activity is both valid and necessary. 



After pointing out in the lecture that physical science still 

 professes the principles of Kant (whose philosophy does not 

 add to the content of cognition by pure thought, but derives 

 all perception of reality from experience, and makes the 

 sources of our knowledge and the degree of its justification 

 the sole objects of investigation), he proposes the Theory of 

 Sense-Perception in man as the real theme of his lecture, 

 since it is here that philosophy and natural science are most 

 in touch. He inquires how the empirical data for the organ 

 of the eye stand in relation to the philosophical theory of 

 knowledge. After a full account of the construction of the 

 eye, and of his theory of accommodation, he gives an explana- 

 tion of Joh. Miiller's fundamental doctrine of specific senses, 

 ' Light is only light when it falls on the seeing eye/ The 

 discussion of the theory of colours, the facts on which the 

 construction of the stereoscope is based, and other optical 

 phenomena, show us more and more plainly how little we 

 reflect in the daily, practical use of our sense-organs on the 

 part these have to play, how exclusively we interest ourselves 

 in such perceptions as bring us intelligence from the outer 

 world, and how little we attend to other perceptions not 

 adapted to this end. Now as consciousness (contrary to the 

 earlier theories) does not perceive sensations locally, at their 

 seat in the body, it can only know by unconscious inference 

 whatever we do not perceive directly. This inference is 



