Predictability is a characteristic of law-governed phenomena.

When the laws are expressible as mathematical functions of time, knowledge of the initial conditions at some time allows us to predict the conditions at all later (and retrospectively earlier) times.

The Marquis de Laplace imagined an intelligent being (Laplace's Demon) who knows the positions and velocities of the constituent atoms and uses Newton's equations of motion to predict the future of the entire universe.

Gootfried Leibniz imagined a scientist who could see the events of all times, just as all times are thought to be present to the mind of God. "Everything proceeds mathematically...if someone could have a sufficient insight into the inner parts of things, and in addition had remembrance and intelligence enough to consider all the circumstances and take them into account, he would be a prophet and see the future in the present as in a mirror."

Predictability is also said to be a requirement for natural science. It is related to the idea of reproducibility. In order for an experiment to be accepted as scientific evidence, the observational results must be reproducible and repeatable.

However every scientist knows that predicting the outcome of an experiment is one thing, accomplished by theories and deductions.

Confirming a prediction is another thing. Establishing quantitative observational agreement with the prediction is prone to unavoidable observational errors.

Even before quantum uncertainty and indeterminacy, some practical philosophers understood that chance was involved in all physical experiments. ¹

[Predictability must be disambiguated from its close relatives causality, determinism, necessity, and certainty.]