Letters 113.18

Every living thing possessed of reason is inactive if it is not first stirred by some external impression; then the impulse comes, and finally assent confirms the impulse. Now what assent is, I shall explain. Suppose that I ought to take a walk: I do walk, but only after uttering the command to myself and approving this opinion of mine. Or suppose that I ought to seat myself; I do seat myself, but only after the same process. This assent is not a part of virtue.

Seneca, Letters to Lucilius 113.18, translated by Richard Mott Gummere