Do what you love. Know your own bone; gnaw at it, bury it, unearth it, and gnaw it still. Do not be too moral. You may cheat yourself out of much life so. Aim above morality. Be not simply good, be good for something.

Henry David Thoreau (1817-1862) American philosopher and writer

Letter (1848-03-27) to Harrison Blake
    (Source)

Collected in F. B. Sanborn, ed., Familiar Letters (1865),