Liberty or free will, then, does not depend on actions being disconnected from their motives. Rather, it means simply that actions depend on determinations of the will. Liberty, then, should be contrasted with constraint--the inability to obey one's own will--rather than with necessity.
The debate regarding the compatibility of free will (liberty) and determinism (necessity) has a long and distinguished history in ethics, and is perpetuated even today. The question under debate is how we can reconcile, on the one hand, the view that all events are causally necessitated (determinism) and the on the other hand, the view that in any given situation, a person could have behaved otherwise (free will). If all events are causally necessitated, argues that incompatibilist, then human actions must also be causally necessitated, and if human actions are causally necessitated, then people could not possibly behave other than they do. Thus, the incompatibilist argues, free will and determinism are incompatible (hence a proponent of this argument is called an "incompatibilist").
The incompatibilist picture raises some serious issues for either metaphysics or ethics. Hard determinists reject the idea that humans have free will, which raises the ethical question of how we can be held responsible for our actions. If I could not have behaved otherwise, how can you blame me for my actions? Libertarians reject the idea that determinism is true, which raises serious metaphysical problems regarding necessity and order in the universe.
Hume places himself firmly in the compatibilist camp on the free will/determinism debate, arguing that the two notions can be reconciled. Such a position relies on defining free will and determinism in such a way as to avoid the logic of the incompatibilist position.
Hume's trick lies in altering our conception of a deterministic universe. According to Hume, the incompatibilist picture of determinism claims the existence of causation or necessary connection in physical interactions that we deny exist in human behavior. In previous sections, Hume has argued quite forcefully against this picture, suggesting instead that we can observe only constant conjunction, and not necessary connection, in nature. Our idea of necessity derives only from a determination in our thoughts to perceive two events as connected. Thus, for Hume, determinism ceases to rely on events being causally necessitated, and relies only on our perception of them as being causally necessitated.
Hume also redefines free will in order to render it compatible with this new conception of determinism. Rather than contrast it with determinism as the freedom to have acted otherwise, Hume contrasts it with constraint as the ability to act in accordance with one's will. An action is free, not if it could have been otherwise (which raises the metaphysical question of what is meant by this "could") but if we can claim that the action was performed in accordance with our will, if we can say "I chose to do x."