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. The author shows that the thesis is in fact ambiguous between three substantially different theses. She illustrates the importance of clarifying it by assessing its use in the debate against the existence of a priori knowledge. She shows how the three different readings of the thesis can be used to generate three substantially different and philosophically significant arguments against the a priori. The author further challenges each one of these arguments against the a priori.