No Crystal Balls

A crystal ball, in Hall’s (1994) sense, need be neither round nor crystalline. The world is said to contain crystal balls whenever the present carries news of the as-yet-undetermined parts of the future. Images appearing in spheres made of magical quartz might be crystal balls, in the relevant sense, but so too might arrangements of magical tealeaves or neural states in the brains of time travelers or clairvoyants. Many philosophers believe that crystal balls are metaphysically possible.1 In this essay, I argue that they are not. Whether crystal balls are possible matters, for at least two reasons. The first is epistemological. According to a simple, user-friendly chance norm for credence, which I call the Present Principle, agents are rationally required to conform their credences to their expectations of the present chances, deferring to the present chances as they would to an expert. I would like to defend the Present Principle since its truth would do much to simplify the relation between chance and credence. But the Present Principle is counterexample-free, and hence defensible, only if crystal balls are impossible. The second reason is decision-theoretic. The problem of crystal balls is one of the main objections to causal decision theory.2 Critics of causal decision theory, Egan (2007) and Price (2012), claim that crystal ball cases are counterexamples to causal decision theory. But even proponents worry. Lewis, for example, in his 1981 defense of causal decision theory, draws special attention to the problem. He says that crystal ball cases are “much more problematic for decision theory than the Newcomb problems” (1981, 18) and then sets such cases aside, restricting his defense of causal decision theory. I believe that Lewis’s restriction is unnecessary and that the problem of crystal balls can be laid to rest. Crystal ball cases can be counterexample to causal decision theory only if crystal balls are possible, and, as I argue below, crystal balls are not possible.