72 , VISION. 
appropriate organs by those vibrations, should pro- 
duce, one the sensation called sound, and the other 
a sensation altogether incomparable with it, namely 
vision. Neither can we say how it is that the vibra- 
tions of heat produce a sensation different from 
either. In short, beyond the sequence made known 
by experience, we know of no relationship between 
sonorous, luminous, and thermic vibrations on the 
one hand, and the sensations of sound, sight, and 
heat on the other. The sensations may be purely 
arbitrary consequences of the irritations produced 
by the various stimuli, for all that science can tell 
us to the contrary. We perceive in this an instance 
of the impossibility, in our circumstances, of bridging 
the gulf between matter and consciousness. 
While, however, the sensations received through 
the eye, the ear, and the surface of the body, so 
differ in kind as to be incomparable, they yet 
combine, by means of the addition of time and 
space, to give common information. It is ad- 
mitted that time is an idea independent of the 
external senses; and, notwithstanding opinions 
to the contrary, the position is possible that 
the same is true of space, and that the mind 
must from the first recognise itself as sur- 
rounded by xox ego as well as existent. How- 
ever that may be, a sense of the position of the 
