MY CHILD, do not trust in your present feeling, for it will soon give way to another. As long as you live you will be subject to changeableness in spite of yourself. You will become merry at one time and sad at another, now peaceful but again disturbed, at one moment devout and the next indevout, sometimes diligent while at other times lazy, now grave and again flippant.
But the man who is wise and whose spirit is well instructed stands superior to these changes. He pays no attention to what he feels in himself or from what quarter the wind of fickleness blows, so long as the whole intention of his mind is conducive to his proper and desired end. For thus he can stand undivided, unchanged, and unshaken, with the singleness of his intention directed unwaveringly toward Me, even in the midst of so many changing events. And the purer this singleness of intention is, with so much the more constancy does he pass through many storms.
… Thomas à Kempis, “The Imitation of Christ”