On the Happy Life, Section 9 — In Plain English

Seneca — The Senator

But our opponent says, "You only practice virtue because you hope to get some pleasure from it." First, even if virtue does give us pleasure, that's not why we pursue her. She doesn't give this as her main gift — she throws it in as a bonus. This isn't the goal she works toward. She achieves this too, but she's aiming at something else. Think of a farmer plowing his field to grow corn. He might find some flowers mixed in with his crop. These flowers might delight the eye, but he didn't do all that work to grow them. The farmer had another purpose. The flowers were just an extra benefit. That's how pleasure works with virtue. Pleasure isn't the reward or the reason for virtue — it comes along for the ride. We don't choose virtue because she gives us pleasure. But if we do choose her, she gives us pleasure too.

Seneca — The Senator Original

"But," says our adversary, "you yourself only practise virtue because you hope to obtain some pleasure from it." In the first place, even though virtue may afford us pleasure, still we do not seek after her on that account: for she does not bestow this, but bestows this to boot, nor is this the end for which she labours, but her labour wins this also, although it be directed to another end. As in a tilled-field, when ploughed for corn, some flowers are found amongst it, and yet, though these posies may charm the eye, all this labour was not spent in order to produce them—the man who sowed the field had another object in view, he gained this over and above it—so pleasure is not the reward or the cause of virtue, but comes in addition to it; nor do we choose virtue because she gives us pleasure, but she gives us pleasure also if we choose her.