Causal determinism is the thesis that every event—in the past, present, and future—is the result of a combination of causative events ("causes"). Without exception, every event in the present and future is a consequence of events in the past; everything is the necessary result of a sequence of causes.

This may seem to be a simple and obvious thesis, but the consequences of it are significant and recondite. For, if it is true, the events of the past dictate wholly the events of the present and future, thus making these events predictable. To clarify, a hypothetical, sufficiently vast intellect which, at any given moment, knew all the forces that animate the universe and the mutual states and positions of all entities that comprise nature, would know the activity of all bodies in the universe in eternity, with no uncertainty. If the future can thus be predicted with no uncertainty, then the future is as set and unalterable as the past; any future other than the one fixed from eternity is impossible.

This would also entail that the actions of human beings are equally fixed; all of our mental states and actions are causally necessitated and it is impossible for any thought or action to be other than what it is. A person's experiences and memories, the biological and psychological compositions of the brain and of the mind, and the present stimuli that compel the individual to action, are all the necessitated effects of causes.

If free will is the freedom to choose a course of action without the restraint of necessity or coercion, this thesis seemingly forbids free will. If a person's actions are all inevitable, then it seems that it is necessary for a person in a situation to make the choice that he does, and impossible for him to make any other choice. He may deliberate extensively, the deliberation itself a product of causes, but he can only deliberate in a certain predetermined way, and can only conclude by acting in a predetermined manner.

Then, if a person is invincibly directed by causes, how can he be held morally responsible for his actions? Reasonably, a person is not held morally responsible for his actions if he had no choice in his actions. For instance, a person physically forced by another person to kill is not considered responsible for the death; it would not be possible for the person being forced to do anything but follow the function forced on him. There is no choice in the matter. Similarly, how can a person be the object of resentment or commendation if it is impossible for him to deviate from his hated or lauded course?

Alternatively, quantum theory tells us that the subatomic activities of the world are truly random, and possibly that this reality is only a single reality in the realm of infinite possible realities. If this were true, and the random events of the most basic levels may be the causes of all else, then everything, including the actions of a person, would be random. On the other hand, we can see around us that it is highly improbable or impossible for random events to manifest in the macroscopic world, apparently due to statistical smoothing. However, if a person's actions may only be saved from causal necessity by random chance, there is still no moral responsibility.

This view may appear to lead one to fatalism; the invincibility of motivations might move a person to despondency or apathy. However, this need not be so. Despite the impossibility of alternative action, causal necessity may still be reconciled with free will.

On the one hand, all the events of a person's life may be hypothetically predictable from beginning to end, and it would be impossible for a person to do anything other than what he does.

On the other hand, if a person is simply defined at any given moment as a culmination of the past, such a person would have a manner of deliberation and choice that is consistent with his desires. If given the choice, ungoverned by causal necessity, that "person," at a given moment defined by his experience and biology, would make the same decisions as the ones that are causally necessitated. Thus, the actions of a person are still voluntary; a person's will is just not originative or boundless. Thus, it cannot be said that a person is ever unnaturally forced by causes to contradict his wishes, only that, because his wishes are the result of causes, his wishes directly concur with what is causally necessitated.