use or exercise the mind or one's power of reason in order to make inferences, decisions, or arrive at a solution or judgments

great and constant diligence and attention

complete attention; intense mental effort

devote (oneself) fully to

cause one's (or someone else's) thoughts or attention to return from a reverie or digression

focus one's attention on a certain state

examine closely; focus one's attention on

listen and pay attention

hold (someone's attention)