Free will is the philosophical doctrine that holds that our choices are ultimately up to us. Conversely, an unfree action would be "up to" something else. The phrase "up to us" is vague, and, just like free will itself, admits of a variety of interpretations. Because of this vagueness, the usefulness of the concept of free will is questioned by some. Several logically independent questions can be asked about free will.
Determinism vs. indeterminism
Determinism holds that each state of affairs is necessitated (determined) by the states of affairs that preceded it. Indeterminism holds that determinism is false, and that there are events which are not entirely determined by previous states of affairs. The idea of determinism is sometimes illustrated by the story of Laplace's demon, who knows all the facts about the past and present and all the natural laws that govern our world, and uses this knowledge to foresee the future, down to every detail.
Some philosophers hold that determinism is at odds with free will. This is the doctrine of incompatibilism. Incompatibilists generally claim that a person acts freely (has free will) just in cases where the person is the sole originating cause of the act and the person genuinely could have done otherwise. This kind of free will is (at least allegedly) incompatible with determinism. If determinism is true, and everything that happens is completely determined by the past (including events that preceded our births), then every choice we make would ultimately be determined by prior events that were not under our control. Our choices would be just another outcome determined by the past. So if determinism were true, then we would be trapped by the past and free will would be an illusion. "Hard determinists", such as d'Holbach, are those incompatibilists who accept determinism and reject free will. "Libertarians", such as Thomas Reid and Peter van Inwagen, are those incompatibilists who accept free will, deny determinism, and instead believe that indeterminism is true. (This kind of libertarianism should not be confused with the political position of the same name, and is thus sometimes known as voluntarism for this very reason.)
Other philosophers hold that determinism is compatible with free will. These "compatibilists", such as Hobbes, generally claim that a person acts freely only in the case where the person willed the act and the person could (hypothetically) have done otherwise if they had decided to. In articulating this crucial proviso, Hume writes, "this hypothetical liberty is universally allowed to belong to every one who is not a prisoner and in chains". Compatibilists often point to clearcut cases of someone's free will being denied — rape, murder, theft, and so on. The key to these cases is not that the past is determining the future, but that the aggressor is overriding the victim's desires and preferences about his or her own actions. The aggressor is coercing the victim and, according to compatibilists, this is what nullifies free will. In other words, determinism does not matter; what matters is that our choices are the results of our own desires and preferences, and are not overridden by some external (or even internal) force. To be a compatibilist, one needn't endorse any particular conception of free will (one need only deny that determinism is at odds with free will), but the positions canvassed here are typical of compatibilism.
Furthermore, it is often held that the phrase "free will" is, as Hobbes put it, "absurd speech", because freedom is a power defined in terms of the will, which is a thing—and so the will is not the sort of thing that could be free or unfree. Some compatibilists argue that this alleged lack of grounding for the concept of "free will" is at least partly responsible for the perception of a contradiction between determinism and liberty. Also, from a compatibilist point of view the use of "free will" in an incompabilist sense may be regarded as loaded language.
We generally hold people responsible for their actions, and will say that they deserve praise or blame for what they do. However, moral responsibility is believed by many to require free will. Thus, another important issue is whether we are ever morally responsible, and if so, in what sense.
Incompatibilists tend to think that determinism is at odds with moral responsibility. After all, how can you hold someone responsible for an action that could be predicted from the beginning of time? Hard determinists say "So much the worse for moral responsibility!" and junk the concept — Clarence Darrow famously used this argument to defend the murderers Leopold and Loeb — while libertarians say "So much the worse for determinism!" This issue appears to be the heart of the dispute between hard determinists and compatibilists; hard determinists are forced to accept that we often have "free will" in the compatibilist sense, but they deny that this sense of free will truly matters — that it can ground moral responsibility. Just because an agent's choices are uncoerced does not change the fact that determinism robs the agent of responsibility.
Compatibilists often argue that, on the contrary, determinism is a prerequisite for moral responsibility — you can't hold someone responsible unless his actions were determined by something (this argument can be traced to Hume and was also used by the anarchist William Godwin). After all, if indeterminism is true, then those events that are not determined are random. How can you blame or praise someone for performing an action that just spontaneously popped into his nervous system? Instead, they argue, you need to show how the action stemmed from the person's desires and preferences — the person's character — before you start holding the person morally responsible. Libertarians sometimes reply that undetermined actions are not random at all, and that they result from a substantive will whose decisions are undetermined. This move is widely considered unsatisfactory, for it just pushes the problem back a step, and further, it involves some very mysterious metaphysics.
Compatibilist theories and the could-have-done-otherwise principle
Many claim that, in order for a choice to be free in any sense that matters, it must be true that the agent could have done otherwise. They take this principle — van Inwagen calls it the "principle of alternate possibilities" — to be a necessary condition for freedom. For instance, if a criminal puts a machine in Bob's brain that makes him kill a stranger, his action was not free, for Bob couldn't have done otherwise. Incompatibilists often appeal to this principle to show that determinism cannot be reconciled with free will. "If a decision is completely determined by the past," they ask, "how could the agent have decided to do something else?" Compatibilists often reply that what's important is not simply that the agent could have done otherwise, but that the agent could have done otherwise if he or she had wanted to. Moreover, some compatibilists, such as Harry Frankfurt or Daniel Dennett, argue that there are stark cases where, even though the agent couldn't have done otherwise, the agent's choice was still free: what if Bob really wanted to kill the stranger and the machine in Bob's brain would only kick in if Bob lost his nerve? If Bob went through with it on his own, surely the act would be free. Incompatibilists claim that the problem with this idea is that what Bob "wanted" was determined before Bob was conceived. In Elbow Room, Dennett presents an argument for a compatibilist theory of free will.
The philosopher John Locke also took the view that determinism was irrelevant. He believed, however, the defining feature of free will to be that we are free so long as we have the ability to postpone a decision long enough to reflect upon the consequences of a choice.
More sophisticated analyses of compatibilist free will have been offered. A free action may require not only liberty from external coercion (according to some), but also liberty from internal conflicts. Compulsive behaviors and the actions of the insane are thus not free. Moreover, our common sense conceptions of free will also demand the possibility that an agent could act rationally or irrationally with equanimity. In either case, what we mean by free will could be that an agent can claim ownership of his or her will despite external or internal influences.
The science of free will
Throughout the history of science, attempts have been made to answer the question of free will using scientific principles. Early scientific thought often pictured the universe as deterministic, and some thinkers believed that it was simply a matter of gathering sufficient information to be able to predict future events with perfect accuracy. While not mechanistic in the same sense as classical physics, most current scientific theories are also deterministic, by necessity — it is a basic assumption of all scientific endeavors that the future can be predicted. It is also difficult, if not impossible, to write the mathematics for a non-predictive science.
Various interpretations of quantum mechanics may suggest that the universe, when viewed as a single system, is deterministic, as there is no outside entity capable of making observations, aside possibly from God. It is far from clear, however, that microscale interpretations of quantum mechanics can be applied to large systems in this way, and whether quantum mechanics ultimately describes a universe governed by laws of cause and effect or by chance is hotly debated both by physicists and philosophers of science.
Like physicists, biologists have also frequently addressed the question of free will. One of the most heated debates of biology is that of "nature versus nurture". How important are genetics and biology in human behavior compared to culture and environment? Genetic studies have identified many specific genetic factors that affect the personality of the individual, from obvious cases such as Down's syndrome to more subtle effects such as a statistical predisposition towards schizophrenia. However, it is not certain that environmental determination is less threatening to free will than genetic determination. The latest analysis of the human genome shows it to have only about 20,000 genes. The information content of the latter is a mere 2 or 3 megabytes (despite junk DNA, which may really have almost no information content), implying that nurture is more important than genetic determinists used to claim.
It has also become possible to study the living brain and researchers can now watch the decision-making "machinery" at work. A seminal experiment in this field was conducted by Benjamin Libet in the 1980s, wherein he asked subjects to choose a random moment to flick their wrist while he watched the associated activity in their brains. Libet found that the brain activity leading up to the subject flicking their wrist began approximately one-third of a second before the subject consciously decided to move, suggesting that the decision was actually first being made on a subconscious level and only afterward being translated into a "conscious decision".
A related experiment performed later by Dr. Alvaro Pascual-Leone involved asking subjects to choose at random which of their hands to move. He found that by stimulating different hemispheres of the brain using magnetic fields it was possible to strongly influence which hand the subject picked. Normally right-handed people would choose to move their right hand 60% of the time, for example, but when the right hemisphere was stimulated they would instead choose their left hand 80% of the time (recall that the right hemisphere of the brain is responsible for the left side of the body, and the left hemisphere for the right). Despite the external influence on their decision-making, the subjects continued to report that they believed their choice of hand had been made freely. Libet himself (e.g. Libet, 2003: 'Can Conscious Experience affect brain Activity? ', Journal of Consciousness Studies 10, nr. 12, pp 24 - 28), however, does not interpret his experiment as evidence of the inefficacy of conscious free will — he points out that although the tendency to press a button may be building up for 500 milliseconds, the conscious will retains a right to veto that action in the last few milliseconds. A good comparison made is with a golfer, who may swing the club several times before striking the ball. In this view, the action simply gets, as it were, a rubber stamp of approval at the last millisecond. Also, for planning tomorrow's activities or those in an hour millisecond offsets are insignificant.
Neurology and psychiatry
There are several brain-related disorders that might be termed free will disorders: In obsessive-compulsive disorder a patient may feel an overwhelmining urge, e.g., to wash his hands many times a day, and he will recognize the desire as his desire although out of control. In Tourette's and related syndromes patients will involutarily make movements (tics) and utterances. In the alien hand syndrome (Dr. Strangelove syndrome) patient's limb will make meaningful acts without the intention of the subject.
Determinism and emergent behaviour
In emergentist or generative philosophy of cognitive sciences and evolutionary psychology, free will is the generation of infinite behaviour from the interaction of finite-deterministic set of rules and parameters. Thus the unpredictablility of the emerging behaviour from deterministic processes leads to a perception of free will, though free will as an ontological entity does not exist.
As an illustration, the strategy board-games chess and more so Go are probably the most rigorous and deterministic in its rules and parameters in terms of the positions of the pieces or entities in relation to other entities on the board. Yet, chess and especially Go with its extreme rigour and rules, generates more moves and unpredictable behaviour than any other games in existence. By analogy, emergentists or generativists suggest that the experience of free will emerges from the interaction of finite rules and deterministic parameters that generate infinite and unpredictable behaviour.
Dynamical-evolutionary psychology, cellular automata and the generative sciences, model emergent processes of social behaviour on this philosophy, showing the experience of free will as essentially a gift of ignorance or as a product of incomplete information.
The theological doctrine of divine foreknowledge is often alleged to be in conflict with free will. After all, if God knows exactly what will happen, right down to every choice one makes, how can one's choices be free? God's already true or timelessly true knowledge about one's choices seems to constrain one's freedom. This problem is related to the Aristotelian problem of the sea-battle: tomorrow there will or will not be a sea-battle. If there will be one, then it was true yesterday that there would be one. Then it would be necessary that the sea battle will occur. If there won't be one, then by similar reasoning, it is necessary that it won't occur. This means that the future, whatever it is, is completely fixed by past truths — true propositions about the future. (However, some philosophers hold that necessity and possibility are defined with respect to a given point in time and a given matrix of empirical circumstances, and so something that is merely possible from the perspective of one observer may be necessary from the perspective of an omniscient.)
In Christian theology, God is described as not only omniscient but omnipotent, which some people (Christians and non-Christians alike) believe implies that not only has God always known what choices you will make tomorrow, but actually chose what you would choose. That is, they believe, by virtue of His foreknowledge He knows what will influence your choices, and by virtue of His omnipotence He controls those factors. This becomes especially important for the doctrines relating to salvation. Christians' definition of predestination does not imply that God chose certain people to receive salvation and the rest have no chance of salvation (John 3:16 says "whosoever believeth in him should not perish, but have everlasting life."). Calvinists, however, embrace the idea that God chose them and only them for salvation. Arminians believe that humans always have free will, but God's prevenient grace is always calling them. Mormons believe that God has given all humans the gift of free will (or free agency in Mormon terms) and has also predestined or foreordained everyone to do certain things in life, including to return to his presence. Whenever an individual chooses to stray from the commandments of God, by their own free will, their predestination may be anulled.
Some philosophers believe that free will is equivalent to having a soul, and thus that (at least some) animals don't have free will. This is also the position of Jewish philosophy, which stresses that free will (Hebrew: bechirah chofshith) is a product of the intrinsic human soul (neshama). There is some controversy on the contradiction between God's omniscience and free will; this was first debated between Maimonides and his critic Abraham ibn Daud (Raavad III), e.g. in Maimonides' Mishneh Torah Hilchoth Teshuva 5:5.