deduction from a
theory, that is true if the theory is
This is the basic way to test
any theory: 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.
Note also that any genuine prediction (1) goes beyond the available
evidence and (2) can only be tested if some assumptions are accepted
(concerning invariance or
Finally, it is important to realize that any theory, however plausible it may sound, that does not allow the deduction from it of predictions, and preferably such as only that theory implies, cannot be tested at all, or cannot be tested in a way that confirms only it, if its prediction is verified.
This means that such a theory is not properly testable, and either not empirical at all, for making no empiircally testable predictions whatsoever, or else only in a somewhat imprecize sense, in case it cannot be itself specifically confirmed.
This is the case with many religious claims; most politics; and considerable parts of the social sciences, including economics.