Letters 76.11

Hence that in man is alone a good which alone belongs to man. For we are not now seeking to discover what is a good, but what good is man’s. And if there is no other attribute which belongs peculiarly to man except reason, then reason will be his one peculiar good, but a good that is worth all the rest put together. If any man is bad, he will, I suppose, be regarded with disapproval; if good, I suppose he will be regarded with approval. Therefore, that attribute of man whereby he is approved or disapproved is his chief and only good.

Seneca, Letters to Lucilius 76.11, translated by Richard Mott Gummere