On the Happy Life, Section 9 — In Plain English

Seneca — The Senator

The highest good is found in choosing virtue itself. It's found in the mindset of noble people who have done their job and settled into their proper place. Once they reach this point, they have achieved the highest good and need nothing more. There is nothing outside of wholeness, just as there is nothing beyond the end.

So you're wrong when you ask me what I'm trying to get by pursuing virtue. You're looking for something higher than the highest thing. Do you ask what I want from virtue? I'll tell you: virtue herself. She has nothing better to offer. She is her own reward.

Seneca — The Senator Original

The highest good lies in the act of choosing her, and in the attitude of the noblest minds, which when once it has fulfilled its function and established itself within its own limits has attained to the highest good, and needs nothing more: for there is nothing outside of the whole, any more than there is anything beyond the end. You are mistaken, therefore, when you ask me what it is on account of which I seek after virtue: for you are seeking for something above the highest. Do you ask what I seek from virtue? I answer. Herself: for she has nothing better; she is her own reward.