W. V. Quine famously claimed that no statement is immune to revision. This thesis has had a profound impact on twentieth century philosophy, and it still occupies centre stage in many contemporary debates. However, despite its importance it is not clear how it should be interpreted. I show that the thesis is in fact ambiguous between three substantially different theses. I illustrate the importance of clarifying it by assessing its use in the debate against the existence of a priori knowledge. (...) I show how the three different readings of the thesis can be used to generate three substantially different and philosophically significant arguments against the a priori. I further challenge each one of these arguments against the a priori. (shrink)