It receives some of its most famous formulations in Hume, and it has its most sophisticated formulation in decision theory. Actions, where rational, are caused by beliefs and desires.
Rationality is a matter of following rules of rationality.
The capacity for rationality is a separate faculty in the mind, much as perception is a separate faculty. Where the reasons for an action are properly constructed, then the action must follow by causal necessity. Weakness of will can only occur where there is something wrong with the antecedents of the action. All actions must be motivated by primary desires, where desires are broadly construed to include such things as moral evaluations. The set of initial or primary desires or preferences must be consistent in order for rationality to operate.