Humans everywhere have what Immanuel Kant called "The Idea of Freedom" in his great work Groundwork of the Metaphysic of Morals. Kant based his "categorical imperatives" - without which morality and responsibility would be impossible - on the presumed fact that freedom is a universal idea.

Now determinists generally deny both freedom and moral responsibility, while compatibilists generally assert a special form of free will compatible with determinism that allows them to defend responsibility.

But note the curious fact that all the participants in the free will debates are in basic agreement with Kant that there exists an Idea of Freedom, even as some deny that there is something phenomenally and physically real corresponding to the Idea and others redefine the meaning of the term "free will".

Very subtle logical arguments, common today among many philosophers, claim that this common sense notion of a "free will" is an illusion. This would seem to require that the burden of proof should be on the shoulders of those determinists, compatibilists, and others who deny the existence of free will in the ordinary common sense usage (that alternative possibilities for action not only exist but can be generated as needed by agents).

Until the mid-twentieth century, compatibilists generally accepted the need for alternative possibilities, but followers of Harry Frankfurt have developed sophisticated arguments to show the "compatibilist free will" does not require the existence of real alternative possibilities.

Buoyed by widespread acceptance of so-called "Frankfurt Cases" in the philosophical literature, some compatibilists now claim that the burden of proof has shifted back to the libertarians to make an intelligible case for how indeterminism does not make our actions random and destroy our responsibility for them.

Some scientists, by contrast, have asserted a "free will axiom, that science would be impossible if investigators were not free to choose - and to invent - both their hypotheses and their experimental tests. Other scientists claim that deterministic causal laws are the sine qua non of physical science. (René Thom calls the "fascination with chance an antiscientific attitude par excellence")

THE CONCEPT OF FREEDOM IS THE KEY TO EXPLAIN AUTONOMY OF THE WILL

WILL is a kind of causality belonging to living beings so far as they are rational. Freedom would then be the property this causality has of being able to work independently of determination by alien causes; just as natural necessity is a property characterizing the causality of all non-rational beings — the property of being determined to activity by the influence of alien causes.

The above definition of freedom is negative and consequently unfruitful as a way of grasping its essence; but there springs from it a positive concept, which, as positive, is richer and more fruitful. The concept of causality carries with it that of laws (Gesetze) in accordance with which, because of something we call a cause, something else — namely, its effect — must be posited (gesetzt). Hence freedom of will, although it is not the property of conforming to laws of nature, is not for this reason lawless: it must rather be a causality conforming to immutable laws, though of a special kind; for otherwise a free will would be self-contradictory. Natural necessity, as we have seen, is a heteronomy of efficient causes; for every effect is possible only in conformity with the law that something else determines the efficient cause to causal action. What else then can freedom of will be but autonomy, that is, the property which will has of being a law to itself? The proposition 'Will is in all its actions a law to itself' expresses, however, only the principle of acting on no maxim other than one which can have for its object itself as at the same time a universal law. This is precisely the formula of the categorical imperative and the principle of morality. Thus a free will and a will under moral laws are one and the same.

Consequently if freedom of the will is presupposed, morality, together with its principle, follows by mere analysis of the concept of freedom. Nevertheless the principle of morality is still a synthetic proposition, namely: 'An absolutely good will is one whose maxim can always have as its content itself considered as a universal law'; for we cannot discover this characteristic of its maxim by analysing the concept of an absolutely good will. Such synthetic propositions are possible only because two cognitions are bound to one another by their connexion with a third term in which both of them are to be found. The positive concept of freedom furnishes this third term, which cannot, as in the case of physical causes, be the nature of the sensible world (in the concept of which there come together the concepts of something as cause and of something else as effect in their relation to one another). What this third term is to which freedom directs us and of which we have an Idea a priori, we are not yet in a position to show here straight away, nor can we as yet make intelligible the deduction of the concept of freedom from pure practical reason and so the possibility of a categorical imperative: we require some further preparation.

FREEDOM MUST BE PRESUPPOSED AS A PROPERTY OF THE WILL OF ALL RATIONAL BEINGS

It is not enough to ascribe freedom to our will, on whatever ground, unless we have sufficient reason for attributing the same freedom to all rational beings as well. For since morality is a law for us only as rational beings, it must be equally valid for all rational beings; and since it must be derived solely from the property of freedom, we have got to prove that freedom too is a property of the will of all rational beings. It is not enough to demonstrate freedom from certain alleged experiences of human nature (though to do this is in any case absolutely impossible and freedom can be demonstrated only a priori): we must prove that it belongs universally to the activity of rational beings endowed with a will. Now I assert that every being who cannot act except under the Idea of freedom is by this alone — from a practical point of view - really free; that is to say, for him all the laws inseparably bound up with freedom are valid just as much as if his will could be pronounced free in itself on grounds valid for theoretical philosophy.* And I maintain that to every rational being possessed of a will we must also lend the Idea of freedom as the only one under which he can act. For in such a being we conceive of a reason which is practical — that is, which exercises causality in regard its objects. But we cannot possibly conceive of a reason as being consciously directed from outside in regard to its judgement, for in that case the subject would attribute the determination of his power of judgement, not to his reason, but to an impulsion. Reason must look upon itself as the author of its own principles independently of alien influences. Therefore as practical reason, or as the will of a rational being, it must be regarded by itself as free; that is, the will of a rational being can be a wi11 of his own only under the Idea of freedom, and such a will must therefore — from a practical point of view — be attributed to all rational beings.

*This method takes it as sufficient for our purpose if freedom is presupposed merely as an Idea by all rational beings in their actions; and I adopt it in order to avoid the obligation of having to prove freedom from a theoretical point of view as well. For even if this latter problem is left unsettled, the same laws as would bind a being who was really free are equally valid for a being who cannot act except under the Idea of his own freedom. In this way we can relieve ourselves of the burden which weighs upon theory.

(Groundwork of the Metaphysic of Morals, H.J.Paton trans., Harper Torchbook, 1964, pp.114-116.)

Most compatibilists (and even some determinists) admit that on introspection they find that (despite their refined theoretical positions) as a practical matter they continue to deliberate over alternative possibilities, they continue to act on an Idea of Freedom.

Compatibilists argue that determinism is compatible with human freedom, and that indeterminism is not compatible or at best incoherent. If our actions were indeterministic, we could not be responsible for them. (This is a critical part of the standard argument against free will). They feel (correctly) that there must be a deterministic or causal connection between our will and our actions. This, they say, allows us to take responsibility for our actions, including credit for the good and blame for the bad.

As long as the agent is free from external coercion, they have freedom of action, which is the compatibilist freedom we have according to Thomas Hobbes and David Hume.

Compatibilists (or "soft determinists" as they have been known since William James) identify free will with freedom of action - the lack of external constraints. We are free, and we have free will, if we are not in physical chains. But freedom of the will is different from freedom of action.

Some compatibilists accept the view of a causal chain of events going back indefinitely in time, consistent with the laws of nature, with the plan of an omniscient God, or with other determinisms. As long as our own will is included in that causal chain, we are free, they say. And they think causality in nature is related to the very possibility of reason and logic. Without causality, they say, we could not be certain of the truths of our arguments.

Compatibilists don't mind all their decisions being caused by a metaphysical chain of events, as long as they are not in physical chains.

We think compatibilists should be classified according to the particular determinisms they think are compatible with human freedom. It is one thing to claim compatibility with physics, another to claim compatibility with God's foreknowledge, etc.

Other compatibilists, Daniel Dennett, for example, simply insist that such genuine irreducible randomness is not needed for evolution or for human freedom. Others point out that even if strict determinism were true (which it isn't), compatibilist freedom of action, in David Hume's sense, would still exist.

Quantum events introduce the possibility of accidents, novelty, and human creativity. Compatibilists who admit such indeterminism might very likely be convinced of a stronger argument for human freedom that still provides an adequately determined will.

See Giving Determinists What They Say They Want.