The transparency of mental content revisited Paul Boghossian Published online: 15 September 2010 Ó Springer Science+Business Media B.V. 2010 1 Our residual Cartesianism Robert Stalnaker has written a short but extraordinarily rich book, one that sheds light on a number of important and difficult issues on the philosophy of mind. Its overarching claim is that a Cartesian view of the mind continues to color our conception of a range of philosophical issues, even as mainstream thought in the philosophy of mind has tended to move away from Cartesianism. In his book, Stalnaker aims to provide a more thoroughly externalist view of the mental, one which, he claims, will defuse some of the puzzles to which our residual Cartesianism gives rise. One of the puzzles to which Stalnaker applies his general strategy is the one generated by the need to have the contents in the mind satisfy a thesis that I labeled (following Dummett’s closely related thesis about linguistic meaning) transparency. This thesis has two parts, the ‘‘transparency of sameness’’ and the ‘‘transparency of difference.’’ (a) If two of a thinker’s token thoughts possess the same content, then the thinker must be able to know a priori that they do; and (b) If two of a thinker’s token thoughts possess distinct contents, then the thinker must be able to know a priori that they do. 1 I made two main claims about transparency. First, that when mental contents violate one or both of these transparency theses, we get cases in which a thinker who P. Boghossian (&) Department of Philosophy, New York University (NYU), 5 Washington Place, New York 10003, USA e-mail: pb3@nyu.edu 1 Boghossian (1994, p. 36). I am using ‘‘a priori’’ in this connection to mean ‘‘independent of outer experience’’ and therefore in a way that’s consistent with knowledge being a priori if it’s based on inner experience or introspection. 123 Philos Stud (2011) 155:457–465 DOI 10.1007/s11098-010-9611-3