We are conscious of an animal in us, which awakens in proportion as our higher nature slumbers. It is reptile and sensual, and perhaps cannot be wholly expelled; like the worms which, even in life and health, occupy our bodies. Possibly we may withdraw from it, but never change its nature. I fear that it may enjoy a certain health of its own; that we may be well, yet not pure.
The Writings of Henry David Thoreau (ed. 1906)