The Essence of Wisdom
Since no knowledge is better than that by which a man knows himself, let us examine our thoughts, words, and deeds. For what does it avail us if we are to investigate carefully and understand rightly the nature of all things, yet do not understand ourselves?
Liber de Spiritu et Anima, LI. (Migne, P.L., vol. 40, cols. 816-7) (Author unknown)
Quoted in Jung, Aspects of the Masculine, p. 171 note 14.