And now have I not told you that what you mistake for madness is but over-acuteness of the senses? — 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 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.
The Tell-Tale Heart (1843)