P58: Apart from the Joy and Desire that are passions, there are other affects of Joy and Desire that are related to us insofar as we act.

Dem.: When the Mind conceives itself and its power of acting, it rejoices (by P53).

But the Mind necessarily considers itself when it conceives a true, or adequate, idea (by IIP43). But the Mind conceives some adequate ideas (by IIP40 S2). Therefore, it also rejoices insofar as it conceives adequate ideas, i.e. (by P1), insofar as it acts.

Next, the Mind strives to persevere in its being, both insofar as it has clear and distinct ideas and insofar as it has confused ideas (by P9). But by striving we understand {NS: here} Desire (by P9 S). Therefore, Desire also is related to us insofar as we understand, or (by P1) insofar as we act, q.e.d.