Letters 121.3

And when I seek the reason why Nature brought forth man, and why she set him above other animals, do you suppose that I have left character-study in the rear? No; that is wrong. For how are you to know what character is desirable, unless you have discovered what is best suited to man? Or unless you have studied his nature? You can find out what you should do and what you should avoid, only when you have learned what you owe to your own nature.

Seneca, Letters to Lucilius 121.3, translated by Richard Mott Gummere