Regret is the desire or appetite to possess something, kept alive by the remembrance of the said thing, and at the same time constrained by the remembrance of other things which exclude the existence of it.
Explanation--When we remember a thing, we are by that very fact, as I have already said more than once, disposed to contemplate it with the same emotion as if it were something present; but this disposition or endeavour, while we are awake, is generally checked by the images of things which exclude the existence of that which we remember.

Thus when we remember something which affected us with a certain pleasure, we by that very fact endeavour to regard it with the same emotion of pleasure as though it were present, but this endeavour is at once checked by the remembrance of things which exclude the existence of the thing in question.

Wherefore regret is, strictly speaking, a pain opposed to that of pleasure, which arises from the absence of something we hate (cf.III.xlvii.

note).

But, as the name regret seems to refer to desire, I set this emotion down, among the emotions springing from desire.
XXXIII.