And have I not told you that what you mistake for madness is...

And have I not told you that what you mistake for madness is but over-acuteness of the sense? --now, I say, there came to my ears a low, dull, quick sound, such as a watch makes when enveloped in cotton. I knew that sound well, too. It was the beating of the old man's heart. It increased my fury, as the beating of a drum stimulates the soldier into courage.