Why, the truths a man carries about with him are his tools
 Oliver Wendell Holmes Sr., The Autocrat of the Breakfast-Table (1858). copy citation

Context

“He must be a poor creature that does not often repeat himself. Imagine the author of the excellent piece of advice, “Know thyself,” never alluding to that sentiment again during the course of a protracted existence! Why, the truths a man carries about with him are his tools; and do you think a carpenter is bound to use the same plane but once to smooth a knotty board with, or to hang up his hammer after it has driven its first nail? I shall never repeat a conversation, but an idea often.” source