From the Treatise on Original Sin – 1659
Alas, marvel not at it, these serpents and toads were a long while breeding in the imagination. The pleasure or profit of such a sin was often fancied before. It was again and again committed in thy thoughts before it was expressed in thy life, so that a man can never live unblameably in his life that doth not keep his imagination pure and clean. Hence you have so often evil thoughts complained of as the root of all bitterness, Jer. 4:14. “How long shall thy vain thoughts lodge within thee?” Mat. 15:19. “Out of the heart proceed evil thoughts.” As exhalations and vapors ascending from the earth, which are scarce perceptible, yet at last are congealed into thick and dismal clouds, so those sins which while in the thoughts and imagination were scarce taken notice of, do at last grow into soul and enormous transgressions.