Philosophy
The Metaphysics of Chance and Objective Probability
Quick fact
The atomic bomb's criticality calculations rely on objective chance: when a uranium nucleus fissions, the exact number of neutrons emitted is genuinely random according to quantum theory, not merely unknown.
Why this is interesting
You probably think of probability as a way to describe uncertainty. But what if some probabilities are real features of the world, as real as atoms?
Read the full explanation
Understanding The Metaphysics of Chance and Objective Probability
We often say a coin has a 50% chance of landing heads. But if the coin is tossed under identical conditions, is that probability something 'out there' or just a measure of what we don't know? The metaphysics of chance asks exactly this. To begin, think of a fair coin: the physics of a coin toss is deterministic—if we knew all the forces exactly, we could predict the outcome. So the '50%' only reflects our ignorance. But consider quantum physics: a radioactive atom's decay time seems truly unpredictable on any theory. There, probability might be built into the world. The key distinction: epistemic probability is about our knowledge, while objective chance is about the world itself, independent of what anyone knows.
A deeper explanation
Objective chance, if it exists, is a property of a physical situation: it fixes how often an outcome would occur if the situation were repeated. But how can a single event have a chance? Philosophers have proposed several mechanisms. The frequentist says chance just is the long-run frequency observed. The propensity theory says chance is a propensity or tendency of the system to produce a given outcome—an irreducible physical disposition. The Humean best-system analysis, tied to David Lewis, sees chances as the compact, simple laws that best summarize the actual mosaic of events—the system that balances simplicity and strength. Crucially, objective chance matters because it underwrites our best scientific theories: quantum mechanics gives probabilities for outcomes, and those probabilities seem to be all there is. If objective chance exists, then the world is not fully deterministic—some events occur without a sufficient cause. That has profound implications for our understanding of reality and for debates about free will. Yet, some philosophers, like Lewis, tried to reconcile chance with determinism, but they found contradictions, leading others to accept that objective chance implies indeterminism. The metaphysics of chance, therefore, is not just about numbers but about the fundamental structure of causation and time itself.