The Open Universe consists of the main essay, which takes up slightly less than two thirds of the book, and then three appendices which partly extend the main essay's argument and partly link it to the topic of reductionism. While the entire thing is called a 'postscript' to the Logic of Scientific Discovery, there are relatively few references to that work, and many more to Popper's later books and essays.

The book's main argument is an argument against what Popper calls 'scientific' determinism -- complete with the square quotes. This is the claim that an ideal scientist could predict with any desired accuracy any future event. An ideal scientist is someone who may transcend our limitations, but only in quantity, not in kind. Thus, an ideal scientist can make very fast computations, but not infinitely fast ones; must be thought of as inside the universe; can only gain information about events in his/her own past light cone; and so on. Given all these limitations, one wonders if anyone has ever signed up to 'scientific' determinism. (Popper claims that Laplace wanted us to understand his demon in this way, but this seems clearly false.) Popper is interesting and inventive when he attacks this form of determinism, but the reader is left to wonder how much is being accomplished.

When he later turn his attention to the more familiar metaphysical determinism, Popper seems to believe that his earlier attack on 'scientific' determinism has taken away any reason to use science as an argument for metaphysical determinism. That is, he claims to have undercut this series of inferences: our theories are deterministic -> 'scientific' determinism is true -> metaphysical determinism is true. But it seems to me that the second step is superfluous. Surely, it is from the fact that our best theories are deterministic that we immediately conclude that the world is deterministic? (At least, that is what the determinist does.) I suspect Popper's reasoning is dependent on his own anti-inductivist philosophy of science, but he does not spell this out.

Throughout, the book is characterised by a combination of fecundity of ideas and laxness in working them out. This is particularly true in Popper's discussion of his Worlds 1, 2 and 3 in the first addendum. These worlds are supposed to carry a lot of weight in metaphysical argumentation, but their status is left entirely unclear. World 3, the world of theories, is supposed to be brought into existence by humans beings, but also has autonomy. How is that possible? Do theories appear in the world when they are invented? Or only when they are believed? Do they ever disappear? What is the ontological status of these entities? Popper says nothing about it. Or again, in the discussion about reduction of the last two addenda, Popper somehow fails to define what he means by 'reduction', leaving the reader unable to evaluate his rather mystifying claim that chemistry cannot be reduced to physics because chemistry involves questions about how the elements were formed by cosmological processes.

All in all, an interesting but flawed defence of indeterminism. ( )