IV DESCARTES' DISCOURSE ON METHOD 173 



" qualities " of the marble ; and it sounds, at first, 

 the height of absurdity to say that all these 

 qualities are modes of our own consciousness, 

 which cannot even be conceived to exist in the 

 marble. But consider the redness, to begin with. 

 How does the sensation of redness arise ? The 

 waves of a certain very attenuated matter, the 

 particles of which are vibrating with vast rapidity, 

 but with veiy different velocities, strike upon the 

 marble, and those which vibrate with one particu- 

 lar velocity are thrown off from its surface in all 

 directions. The optical apparatus of the eye 

 gathers some of these together, and gives them such 

 a course that they impinge upon the surface of 

 the retina, which is a singularly delicate apparatus 

 connected with the termination of the fibres of 

 the optic nerve. The impulses of the attenuated 

 matter, or ether, affect this apparatus and the 

 fibres of the optic nerve in a certain way ; and 

 the change in the fibres of the optic nerve pro- 

 duces yet other changes in the brain ; and these, 

 in some fashion unknown to us, give rise to the 

 feeling, or consciousness of redness. If the 

 marble could remain unchanged, and either the 

 rate of vibration of the ether, or the nature of the 

 retina, could be altered, the marble would seem 

 not red, but some other colour. There are many 

 people who are what are called colour-blind, being 

 unable to distinguish one colour from another. 

 Such an one might declare our marble to be 



