Aristotle is doing something quietly radical here. The epithumiai — appetitive desires, desires for the pleasant — operate by their own momentum, upstream of your reasoned judgment about what would be best. They do not wait for you to finish deliberating. They can move you against the conclusion you have already reached.

This cuts against a story the tradition has been telling since at least the Stoics: that desire, properly understood, reduces to belief, and that correcting the belief corrects the want. If you really knew what was good, you would want it. If you still want the wrong thing, the knowing hasn't gone deep enough yet. The therapeutic hope hidden in that story is considerable — it implies that understanding is curative, that enough insight will finally close the gap between what you know and what you reach for.

Aristotle declines this comfort. The epithumiai flow from representations of the pleasant, not from evaluations of the good, and representations of the pleasant are not dislodged by arguments about the good. The man who knows perfectly well that the drink will ruin his morning still wants the drink — not because his knowledge is defective, but because the desiring faculty takes its instruction from a different source entirely. Akrasia is not an epistemic failure you can think your way out of. It is a structural feature of the soul.