The Cloud of Unknowing, ed. by Evelyn Underhill, [1922], at sacred-texts.com
That a man should weigh each thought and each stirring after that it is, and always eschew recklessness in venial sin.
I SAY not this for that I trow that thou, or any other such as I speak of, be guilty and cumbered with any such sins; but for that I would that thou weighest each thought and each stirring after that it is, and for I would that thou travailedst busily to destroy the first stirring and thought of these things that thou mayest thus sin in. For one thing I tell thee; that who weigheth not, or setteth little by, the first thought—yea, although it be no sin unto him—that he, whosoever that he be, shall not eschew recklessness in venial sin. Venial sin shall no man utterly eschew in this deadly life. But p. 112 recklessness in venial sin should always be eschewed of all the true disciples of perfection; and else I have no wonder though they soon sin deadly. p. 113