The Imitation of Christ, by Thomas a Kempis, tr. by William Benham [1886], at sacred-texts.com
"My Son, trust not thy feeling, for that which is now will be quickly changed into somewhat else. As long as thou livest thou art subject to change, howsoever unwilling; so that thou art found now joyful, now sad; now at peace, now disquieted; now devout, now indevout; now studious, now careless; now sad, now cheerful. But the wise man, and he who is truly learned in spirit, standeth above these changeable things, attentive not to what he may feel in himself, or from what quarter the wind may blow, but that the whole intent of his mind may carry him on to the due and much-desired end. For thus will he be able to remain one and the same and unshaken, the single eye of his desire being steadfastly fixed, through the manifold changes of the world, upon Me.
2. "But according as the eye of intention be the more pure, even so will a man make his way steadfastly through the manifold storms. But in many the eye of pure intention waxeth dim; for it quickly resteth itself upon anything pleasant which occurreth, and rarely is any man found altogether free from the blemish of self-seeking. So the Jews of old came to Bethany, to the house of Martha and Mary, that they might see not Jesus, but Lazarus, whom he had raised from the dead.(1) Therefore must the eye of the intention be cleansed, that it may be single and right, and above all things which come in its way, may be directed unto Me."
(1) John xii. 9.