Ethics, Part III, Proposition 28

We endeavour, as far as possible, to conceive that which we imagine to conduce to pleasure (III. xii.); in other words (II. xvii.) we shall endeavour to conceive it as far as possible as present or actually existing. But the endeavour of the mind, or the mind's power of thought, is equal to, and simultaneous with, the endeavour of the body, or the body's power of action. (This is clear from II. vii. Coroll. and II. xi. Coroll.). Therefore we make an absolute endeavour for its existence, in other words (which by III. ix. note, come to the same thing) we desire and strive for it; this was our first point. Again, if we conceive that something, which we believed to be the cause of pain, that is (III. xiii. note), which we hate, is destroyed, we shall rejoice (III. xx.). We shall, therefore (by the first part of this proof), endeavour to destroy the same, or (III. xiii.) to remove it from us, so that we may not regard it as present; this was our second point. Wherefore whatsoever conduces to pleasure, &c. Q.E.D.