In the philosophy of science of Popper there is no definable measure of the extent to which evidence confirms a hypothesis (see confirmation theory). Instead, hypotheses face the tribunal of experience by surviving efforts to falsify them. The degree of corroboration of a hypothesis by evidence is then a function of the stringency of the test the evidence provides, and hence a measure of the success of the hypothesis in surviving it. Critics have complained that corroboration in this sense is an empty notion, since it provides no reason to trust the hypothesis on any future occasion. See falsifiability, verisimilitude.