While we remain upon earth we are in the Lord’s school, and a principal lesson we have to learn is a knowledge of ourselves, and this can only be attained by a painful experience. Books, sermons, letters cannot teach it, nor can the observations we make on other people lead us far beyond the theory. To have some tolerable ideas of the human heart in general is one thing, to know our own hearts is quite a different thing. The deceitfulness of the heart which we allow in words, enables it to disguise, conceal, and cover its own emotions, so that the supposed sense we have of its deceitfulness is often the very thing that deceives us.
– Letters of Newton, p. 342.