Concepts are the constituents of thoughts, which in turn, are the contents of propositional attitudes. They are also what the predicates of our language express. According to a tradition going back to Plato, questions about comparative content – questions of the form Is concept F the same as concept G? – are purely about relations of ideas, and so are answerable a priori. This does not mean that no experience at all is necessary to answer such questions, for experience may be needed to grasp their content. Call a piece of information about Fs extraneous if it is not required to obtain a proper understanding of the concept F. Then what the traditional assumption says is that no extraneous information is necessary to answer a question about comparative content. Henceforth, I shall refer to this assumption about concepts as the transparency thesis, or T for short.