I should . . . prefer swallowing one incomprehensibility...

I should . . . prefer swallowing one incomprehensibility rather than two. It requires one effort only to admit the single incomprehensibility of matter endowed with thought, and two to believe, first that of an existence called spirit, of which we have neither evidence nor idea, and then secondly how that spirit, which has neither extension nor solidity, can put material organs into motion.