Letter 16 — On Philosophy, the Guide of Life (§6)

“But it is not my purpose now to be led into a discussion as to what is within our own control,—if foreknowledge is supreme, or if a chain of fated events drags us along in its clutches, or if the sudden and the unexpected play the tyrant over us; I return now to my warning and my exhortation, that you should not allow the impulse of your spirit to weaken and grow cold. Hold fast to it and establish it firmly, in order that what is now impulse may become a habit of the mind.”