Let original sin [inherited from Adam] make us walk with continual jealousy and watchfulness over our hearts. The sin of our nature is like a sleeping lion; the least thing that awakens it makes it rage. The sin of our nature, though it seems quiet, and lies as fire hid under the embers, yet if it be a little stirred and blown upon by a temptation, how quickly may it flame forth into scandalous evils? Therefore we had need always to walk watchfully . . . A wandering heart needs a watchful eye.
--Thomas Watson, English Puritan (c. 1620-1686)