Underdetermination is a relation between evidence and theory. More accurately, it is a relation between the propositions that express the (relevant) evidence and the propositions that constitute the theory. The claim that evidence underdetermines theory may mean two things: first, that the evidence cannot prove the truth of the theory, and second, that the evidence cannot render the theory probable. Let us call the first deductive underdetermination and the second inductive (or ampliative) underdetermination. Both kinds of claims are supposed to have a certain epistemic implication, namely that belief in theory is never warranted by the evidence. This is the underdetermination thesis.

Deductive Underdetermination

Deductive underdetermination is pervasive in all interesting cases of scientific theory. If the theory is not just a summary of the evidence, the evidence cannot determine, in the sense of proving, the theory. For instance, no finite amount...