For they are not by themselves the cause of any motion, and when they have received any impression from any external cause, then they can add nothing to it, nor can they subtract anything from it. Moreover, they are out of the reach of any control; for one sensation cannot judge of another which resembles itself; for they have all an equal value. Nor can one judge of another which is different from itself; since their objects are not identical. In a word, one sensation cannot control another, since the effects of all of them influence us equally. Again, the reason cannot pronounce on the senses; for we have already said that all reasoning has the senses for its foundation. Reality and the evidence of sensation establish the certainty of the senses; for the impressions of sight and hearing are just as real, just as evident, as pain.
It follows from these considerations that we ought to judge of things which are obscure by their analogy to those which we perceive