Abstract: This paper elaborates on the relation between Quine's notion of ontological commitment and his philosophy of science. I distinguish and present Quine's solutions to two problems of existence, a semantic problem, roughly amounting to asking how existence can be expressed within a certain language, and an epistemological problem, roughly amounting to how the members of the scientific community can decide which theories are warranted. The gap between these problems is filled by noticing that existence is equated by Quine with reference, and that the principle of context imposes the epistemological primacy of truth over reference. Truth, in its turn, albeit given a purely semantic account along the lines of Tarski, is coextensive with the notion of warranted belief.