P54: The Mind strives to imagine only those things that posit its power of acting.

Dem.: The Mind’s striving, or power, is its very essence (by P7); but the Mind’s essence (as is known through itself) affirms only what the Mind is and can do, not what it is not and cannot do. So it strives to imagine only what affirms, or posits, its power of acting, q.e.d.