Hume turns his considerations of necessary connection toward the topic "Of Liberty and Necessity," the title of section VIII. He suggests that the debate and controversy regarding free will and determinism is simply a matter of the disputants not having properly defined their terms. In fact, he asserts, all people would find themselves of the same opinion on this subject if only they were more careful in their definitions.
Hume begins by examining what we call necessity in physical processes. We are apt to suppose that there are laws in nature that determine the necessary forces, causes, and effects that determine the movements of all bodies without exception. However, as Hume has discussed, our ideas of necessary connection and causation result only from the observation of constant conjunction between events and a certain determination of our minds. We infer the idea of necessary connection, but nowhere observe it directly in nature.
Next, Hume considers human nature and the laws that govern our behavior. Similarly, he finds that throughout history and across cultures our behavior remains relatively constant. Similar motives produce similar actions and similar causes produce similar events. What we call "human nature" springs from a certain regularity that we observe in human behavior in all sorts of circumstances.
If we observe a physical phenomenon that runs counter to our expectations, we will not suppose that the laws of physics have been suspended, but simply that some unobserved and contrary force must also have been acting that upset our predictions. Hume suggests that we might similarly explain the unexpected behavior of people. Rather than perceive them acting at random, we might suppose that there is some hidden motive or unknown personality trait that makes them act contrary to our expectations.
Thus, people, just as much as physical objects, can be understood to behave in accordance with strict laws and principles that we might claim to understand. Everyone would agree that we predict and infer regarding human behavior based on certain observed regularities just as much as we do with dead matter. Our entire behavior is directed by certain expectations of the behavior of others so that, for instance, the farmer would not work the land and put his crops up for sale if he did not expect other people to pay a fair price for them.
Hume suggests that any opposition to this view has sprung from the false supposition that we can perceive necessary connections in nature. Our inferences regarding human nature are based solely on the observation of constant conjunction, and we would strongly deny that any kind of necessity governs our actions. However, if we grant that we observe no necessary connection governing physical phenomena either, we might see that our predictions of human behavior and physical behavior reduce to a similar set of observations. Necessary connection, be it in matter or in human behavior, is not found in the object itself, but in the imagination of the observer.
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."
Hume reconciles free will and determinism by drawing on his deflation of causal necessity that has dominated the Enquiry. This section could be read as a display of the force to which he can put the arguments he has previously set out. If there is no necessary connection, but only constant conjunction, between physical events, then physical events are on a level with human behavior. In both cases we observe certain regularities, make predictions regarding future outcomes, and act according to those predictions. The more our predictions prove correct, the more confident we become in making predictions in that domain. Thus, the difference between our certainty that fire will burn and that liars will deceive is only a matter of degree as to how regular our past observations of fire and liars have been. Fundamentally, our predictions concerning the one are the same kind as our predictions concerning the other.
Hume's conception of human nature, while not necessarily mistaken, is unmistakably rooted in the environment of eighteenth-century Enlightenment thought. Enlightenment thinkers believed firmly in the uniformity and universality of human reason, that all people fundamentally are the same and think the same. Thus, correct reasoning should be universally applicable and heeded. Postmodern thinking tends to hold a far more skeptical and relativistic position toward human reason, suggesting that what might be true for me might not be true for someone else. Hume's assertion that all human behavior follows from the same motives and causes might be held to closer scrutiny today than it was in his day. The relative merits and defects of the Enlightenment view versus the Postmodern view is beyond the scope of this commentary, but can make for some very lively debate between friends.