236 HERMANN VON HELMHOLTZ 



by a persistent activity of memory, which makes it possible 

 to observe and recognize the regular repetitions of such 

 sequences of homogeneous percepts. Hence, even if the 

 qualities of sensation are merely intuitional forms, the sensations 

 themselves being only signs, the specific nature of which 

 depends entirely upon our organization, they still are signs 

 of something that exists or is happening, and thereby sup- 

 ply us with the law of this happening. Conformity in the 

 phenomenal may thus be accepted as unequivocal and actual. 

 If we give the name of substance to that which remains 

 identical, independent of all other things, through all changes 

 of time, and if on the other hand the persistent ratio between 

 alterable magnitudes is the law that binds them together, then 

 this law is all that we can perceive directly, while the concept 

 of substance must for ever remain problematical. * It is only 

 the relations of time, space, equality, and those derived from 

 them, namely those of number, magnitude, and conformity, 

 in brief mathematical relations, which are common to both the 

 outer and the inner world, and in these we can actually strive 

 for complete correspondence of the percepts with the things 

 perceived/ 



From this philosophical basis Helmholtz proceeds to develop 

 his Theory of Space-Perception, constructed from his con- 

 clusions in physiological optics. To the nativistic theory of 

 space-perception as enunciated by Johannes Muller, Helmholtz 

 opposes his empirical theory of vision. According to Muller 

 the retina itself is sensible in its spatial extension, this intuition 

 of space is innate, and the impressions excited from without 

 are referred directly to the corresponding points on the spatial 

 image of the organ : on Helmholtz's view our sensations are no 

 more than signs for external things and processes, which we 

 must learn to interpret by experience and practice. According 

 to the empiricist theory we have to learn the significance of 

 the local signs of sensation (such as are excited by the same 

 colour on different points of the retina), in reference to the 

 external world ; while on the nativistic theory, these local signs 

 are the direct intuition of spatial differences both in kind and 

 in degree. The theory of the stereoscope, simple vision with 

 both eyes, and a long series of other optical phenomena, give 

 1 a remarkable confirmation of the assumption of the empiricist 



