On the Happy Life, Section 7 — In Plain English

Seneca — The Senator

The highest good lasts forever. It never ends and never leaves you sick of it or full of regret. A clear-thinking mind never changes its nature or turns against itself. The best things in life never change. But pleasure dies the moment it delights us most. It has no room to grow, so it quickly becomes tiresome and boring. It fades as soon as its first rush is over. We can't rely on anything that keeps changing by nature. So there can't be anything solid or real in something that comes and goes so fast. Pleasure destroys itself by its very existence — it reaches a point where it stops being pleasure. Even when it's just starting, it's already looking toward its own end.

Seneca — The Senator Original

The highest good is immortal: it knows no ending, and does not admit of either satiety or regret: for a right-thinking mind never alters or becomes hateful to itself, nor do the best things ever undergo any change: but pleasure dies at the very moment when it charms us most: it has no great scope, and therefore it soon cloys and wearies us, and fades away as soon as its first impulse is over: indeed, we cannot depend upon anything whose nature is to change. Consequently it is not even possible that there should be any solid substance in that which comes and goes so swiftly, and which perishes by the very exercise of its own functions, for it arrives at a point at which it ceases to be, and even while it is beginning always keeps its end in view.