Letters 92.1

You and I will agree, I think, that outward things are sought for the satisfaction of the body, that the body is cherished out of regard for the soul, and that in the soul there are certain parts which minister to us, enabling us to move and to sustain life, bestowed upon us just for the sake of the primary part of us. In this primary part there is something irrational, and something rational. The former obeys the latter, while the latter is the only thing that is not referred back to another, but rather refers all things to itself. For the divine reason also is set in supreme command over all things, and is itself subject to none; and even this reason which we possess is the same, because it is derived from the divine reason.

Seneca, Letters to Lucilius 92.1, translated by Richard Mott Gummere