Haifa, born 1953

David Deutsch

Picture the world as one block of spacetime, every moment fixed, and "I could have chosen otherwise" means nothing: there is no other way things go. But physical reality, Deutsch argues, is a multiverse. There really are universes in which you chose the other road, and that is what makes your choice yours.

The view

Deutsch is a physicist at Oxford and one of the founders of quantum computation. He takes quantum theory at its word, in the many-worlds form first proposed by Hugh Everett in 1957: the universe we see is one of a vast number, which split and differentiate wherever quantum events have different outcomes. His books The Fabric of Reality (1997) and The Beginning of Infinity (2011) build a world view on that physics, on the theory of computation, on evolution, and on the philosophy of Karl Popper.

Free will comes up in his chapter on time in The Fabric of Reality, the chapter that our sister site, Philosophy of Time, works through figure by figure. The argument there is that the flow of time is incoherent, and that physics replaced it with spacetime: a single unchanging block in which every moment, including the future, simply sits. Deutsch thinks the block is the right picture of time but the wrong picture of reality, and the difference matters for free will.

The argument

In a single spacetime, nothing could have been otherwise. In the multiverse, something was. After Deutsch, The Fabric of Reality (1997)

Free will, Deutsch notes, is bound up with claims about what would have happened. To say you chose freely is to say, among other things, that if you had judged differently you would have acted differently, and that your choice made a difference to what came after. Logicians call these "if it had been otherwise" statements counterfactuals. In a single deterministic spacetime they have nothing to be about. There is only one history, with no event in which you chose the other road, and so no fact about what would have followed if you had. We can imagine alternatives, but we can imagine any alternative we like.

The multiverse supplies the missing facts. The alternatives are not imagined but real, and they come in definite proportions. There are universes in which you took each road, and it is an objective matter how the versions of you divide between them, and why. Your choice is free, on this view, when that division depends on you: on your deliberation, your values, the reasons you found persuasive. Move the slider below and compare the three pictures.

The figure is our illustration of Deutsch's argument, not one of his diagrams.

The third picture shows why chance is no help. If a quantum coin decided, the versions of you would split half and half whatever your reasons said. You would have done otherwise in many universes, but not because of anything about you. On this picture chance adds nothing to freedom, a point on which Dennett and Popper would agree.

A second strand comes from The Beginning of Infinity. Deutsch adapts a thought experiment from Douglas Hofstadter: a chain of dominoes set up as a computer, which tests whether 641 is a prime number. One domino stays standing. Why? You could trace the path of every falling domino, and every step would be correct. But the explanation is that 641 is prime. The high-level explanation is the real one, and it cannot be replaced by the low-level story without losing what explains. So it is with choices. A choice made for a reason is explained by the reason, even though every atom involved obeys the laws of physics.

Where he meets Dennett

On a great deal. Both are physicalists who think free will is real and needs no ghost. Both refuse to look for it in chance. Both think the explanation of a choice lives at the level of reasons and ideas, not particles: Deutsch's dominoes make the same point as Dennett's real patterns and his Game of Life. Both read evolution and the growth of knowledge through Popper, as trial and the elimination of error. And on "could have done otherwise", they agree on what matters. Dennett's answer to Austin's missed putt was to look at a range of situations very like the actual one and ask what the golfer does across them. Deutsch's multiverse makes that range literal: the very similar situations are real universes, and the question is how the versions of you behave across them.

Where they part

On whether the physics is needed. For Dennett, a single deterministic world is enough. The counterfactuals that matter for freedom are claims about how an agent responds across nearby possibilities, and those claims are true or false in one world, as claims about skills and dispositions always are. Deutsch thinks this leaves counterfactuals with nothing to be true of, and that only a multiverse gives them content. For Dennett, free will does not wait on how quantum theory is interpreted.

They also differ about who has free will. Dennett's freedom comes in degrees, climbing a ladder from bacteria to persons. Deutsch draws a sharp line. People are universal explainers, able in principle to create any explanation there is, and that creativity, which no animal shares, is at the heart of what makes their choices their own. Following Popper, he stresses that the growth of knowledge cannot be predicted, so the future of any creative being is open in a way no other thing's is.

See every view compared →

Further reading

• David Deutsch, The Fabric of Reality (Allen Lane, 1997), the chapter on time, for the multiverse, counterfactuals and free will.
• David Deutsch, The Beginning of Infinity (Allen Lane, 2011), for the dominoes, the reality of abstractions and people as universal explainers.
• Deutsch on time and Deutsch on the mind, on our sister sites.