Although I use the term critical thinking to refer to the general process of hunting and checking assumptions it is not an unequivocal concept, understood in the same way by all who speak or write the term. In fact it is a contested idea. How the term is used reflects the ideology of the user and her disciplinary background. In fact, there are at least four distinct intellectual traditions shaping understandings of critical thinking and these differ substantially, perhaps explaining why so many efforts to teach critical thinking across the curriculum fail so dismally. In rough order of their prominence in the discourse of critical thinking these traditions are (1) analytic philosophy and logic, (2) natural science, (3) pragmatism, and (4) critical theory.