(For the purposes of this question, I am assuming that some form of physicalism holds.)

When I read about hard determinism and compatibilism, it struck me that they seem to be placing different labels on the same diagram. To use more standard terminology, they look (to my eye) like two differently-notated maps of the same territory.

  • Is it possible to distinguish between a hard determinism universe and a compatibilist universe by some physical experiment?
  • If not, what is the objective difference between them? Can one be true and the other false?

Since it has been asked, here are the definitions of these terms that I am familiar with:

  • Hard determinism - The universe's future is either fully determined by its present state, or to the extent that it is not so determined, the nondeterminism manifests as randomness rather than some kind of cognitive process or other purposeful, non-random variation. Free will is thus excluded because there is no non-determinism in which it may exist.
  • Compatibilism - Free will has nothing to do with non-determinism. Our choices are determined by our character, morals, memories, etc., none of which require or even benefit from non-determinism. Purposeful non-determinism is excluded because free will never involved non-determinism, so all non-determinism is purely incidental and non-purposeful (just as in hard determinism).

I have no objection to answers which challenge one or both of these definitions. I am not claiming that they are correct. They are merely the definitions that I happen to know.

  • If there were an experiment that could distinguish the two, this would become physics, not philosophy. A falsifiable theory is normal science.
    – user9166
    Commented Sep 26, 2016 at 5:05
  • @jobermark: Then you have an answer to my second bullet point?
    – Kevin
    Commented Sep 26, 2016 at 5:11
  • In the absence of the possibility of determining their truth via evidence, what would you mean by 'objective'? I will attempt to guess, and give an answer.
    – user9166
    Commented Sep 26, 2016 at 5:17
  • @jobermark: I mean, how can one be right and the other wrong, in some "real" sense?
    – Kevin
    Commented Sep 26, 2016 at 5:22
  • "compatibilism" is actually the name for several views (that share some belief that some conceptualization of the world as deterministic is compatible with some notion of human choice or freedom) rather than a single view. Can you clarify the definition you're using here (specifically for compatibilism -- as physicalism pretty much clears up the source of determinism)?
    – virmaior
    Commented Sep 26, 2016 at 5:36

3 Answers 3


I don't think there is a difference in the "universe" as you phrase it in your question. Compatibilism, as I see it, is more of an attempt to show how free will could (seem to) exist in a deterministic universe, than an attempt to describe a different universe to that of the hard determinists.

  • How can either the compatibilist or the hard determinist claim to be "right" if they are both describing the same thing?
    – Kevin
    Commented Sep 25, 2016 at 17:08
  • 1
    The hard determinists are describing a world in which they are happy to concede there is no free will, the compatibilists are describing the same world (one that is predetermined), but say that in such a world there can be free will because it is the person's motives that are pre-determined, but they have the theoretical freedom to act in any way (they just won't).
    – user22791
    Commented Sep 25, 2016 at 17:25

The argument of 'hard determinism' vs 'compatibilism' is just 'physics + determinism' vs 'physics + free will', and the physics changes enough that it is basically irrelevant to the point. It does not matter whether you argue for or against free by taking physics into account, or by more abstract logic.

Then this is basically Kant's 'Third Antinomy': Spontaneity seems common but should be impossible, with an additional layer of indirection. And it still has the same set of solutions, including his.

He suggests that in antinomies, something is wrong with the question, and both positions make no sense. We need a solution that dissolves the question rather than choosing an answer.

In this case, Kant's resolution is the notion that time is a collective subjective intuition of temporal beings, whereas logical necessity is not.

That makes linking necessity to time, an error. If time is an epiphenomenon of our thinking, and logical necessity is not, then necessity is something far more basic that time (what Kant calls a Category), and the two things cannot be related in either way. Coming later in time than the cause cannot make the future state necessary, nor can time itself be part of the reason other future states remain possible. Necessity or possibility has to cause time, not the other way around, and that makes both determinism and freedom of will non-sequiturs. Both are illusions.

This can be seen as a variety of compatibilism in itself, but it is more of a reason not to bother constructing the theory, and to accept the irrelevance of the reality of free will and accept our intuition of responsibility without taking up a theory on its possible physical truth.

  • I don't think QM is relevant here: a) there are versions of hard determinism (i.e. determinism with regards to the question of freewill) which accommodate quantum randomness, LaPlace's demon is just as demonic even if only calculating wave functions. b) Many physicists believe, contra Penrose, that consciousness is a classical phenomena and entanglement won't have any effect on it, in which case you can't dismiss compatibilism the way you did. Commented Sep 26, 2016 at 6:04
  • @AlexanderSKing I don't deduce anything from the QM. I admit there are theories that reconcile both sides. They still make the relevant positions hard to hold. As I said in the comments, if this could be resolved by physics, it would no longer be philosophy. The point is, space and time as forms of intuition still makes this all silly. It is no less silly once you add the notion of compatibility with physics than it was in the original raw form.
    – user9166
    Commented Sep 26, 2016 at 6:07
  • I'm not sure I understand how Kant is relevant given we're assuming physicalism. I always thought he was an idealist.
    – Kevin
    Commented Sep 26, 2016 at 6:51
  • Entanglement doesn't imply that future affects the past arxiv.org/abs/quant-ph/9906007.
    – alanf
    Commented Sep 26, 2016 at 13:14
  • You don't have to take Kant as a global solution in order to get value out of one or the other of his perspectives.
    – user9166
    Commented Sep 26, 2016 at 13:45

The difference mostly has to do with rewards and punishments and how we define freewill.

With hard determinism, a person who is guilty of a particular law ought to be disiplined. It is the hope that this chastisement (Greek: kolasis, in case you're interested) will cause their underlying behavior to be corrected. If a person does good, they ought to be rewarded in hopes that the reward will cause them to continue acting with good behavior.

With Compatibilism, a person who is guilty of a particular crime deserves to be punished. What they did was wrong, they knew it was wrong, they knew there was another way, but they still chose to commit the crime. Despite the fact that both their will and actions were determined, they were wrong for having that will. If they do good, they deserve to be rewarded because what they did was right, they knew it was right, they knew there was a wrong way, and they still chose what's right. Despite the fact that their will and actions are determined, they were right for having that will. As Arthur Schopenhauer said, "Man can do what he wills but he cannot will what he wills."

A majority of Compatibilism comes from the Reformed sects of Christianity. To them, God has determined all things so that man cannot do otherwise. However, because they have something labeled freewill they deserve to have their flesh burned forever in agonizing pain. How determinism and freewill can be reconciled is usually described as a great mystery.

Other than that, there really is no difference. Both believe that the universe works the exact same way.


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