  Given Evidentialism, various traditional debates within the theory of knowledge are naturally cast as debates about the status of various underdetermination theses. Thus, the skeptic about our knowledge of the external world maintains that one's evidence (understood, perhaps, as the totality of one's present experiences) does not favor one's ordinary, commonsense views about one's surroundings over various skeptical alternatives (e.g., the hypothesis that one is hallucinating in an undetectable way). Similarly, one longstanding controversy that divides realists and antirealists in the philosophy of science can be understood as a debate about whether the kind of evidence which is available to scientists is ever sufficient to justify belief in theories that quantify over entities that are in principle unobservable, such as electrons or quarks.