A holy man will follow after purity of heart. He will dread all filthiness and uncleanness of spirit, and seek to avoid all things that might draw him into it. He knows his own heart is like tinder, and will diligently keep clear of the sparks of temptation. Who shall dare to talk of strength when David can fall? There is many a hint to be gleaned from the ceremonial law. Under it the man who only touched a bone or a dead body or a grave or a diseased person became at once unclean in the sight of God.

Holiness: Its Nature, Hindrances, Difficulties, and Roots (first published 1879).