The concept of identity has been seen to lead to paradox: we cannot truly and usefully say that a thing is the same either as itself or as something else. This book is a full examination of this paradox in philosophical logic, and of its implications for the philosophy of mathematics, the philosphy of mind, and relativism about identity. The author's account involves detailed discussion of the views of Wittgenstein, Russell, Frege, and Hintikka.