Prediction: Observable deduction from a theory, that is true if the theory is true.
This is the basic way to test theories: Derive predictions from it that are not yet known to be true, and find out by experiment if the predictions are true.
Note that there is a subtlety involved if theories are construed probabilistically and may entail probabilities. In that case a theory may be relevant to certain possible facts, in the sense that the probability of the fact given that the theory is true differs from the probability of the fact given that the theory is not true.
