Determinism and indeterminism

Over the centuries, the doctrine of determinism has been understood, and assessed, in different ways. Since the seventeenth century, it has been commonly understood as the doctrine that every event has a cause; or as the predictability, in principle, of the entire future. To assess the truth of determinism, so understood, philosophers have often looked to physical science; they have assumed that their current best physical theory is their best guide to the truth of determinism. It seems that most have believed that classical physics, especially Newton’s physics, is deterministic. And in this century, most have believed that quantum theory is indeterministic. Since quantum theory has superseded classical physics, philosophers have typically come to the tentative conclusion that determinism is false.

In fact, these impressions are badly misleading. The above formulations of determinism are unsatisfactory. Once we use a better formulation, we see that there is a large gap between the determinism of a given physical theory, and the bolder, vague idea that motivated the traditional formulations: the idea that the world in itself is deterministic. Admittedly, one can make sense of this idea by adopting a sufficiently bold metaphysics; but it cannot be made sense of just by considering determinism for physical theories.

As regards physical theories, the traditional impression is again misleading. Which theories are deterministic turns out to be a subtle and complicated matter, with many open questions. But broadly speaking, it turns out that much of classical physics, even much of Newton’s physics, is indeterministic. Furthermore, the alleged indeterminism of quantum theory is very controversial: it enters, if at all, only in quantum theory’s account of measurement processes, an account which remains the most controversial part of the theory.