I am searching for a reference I read and then misplaced, in which Weinberg asserts that the Dirac equation's prediction of the positron is a characteristic of any equation which has the general form of Dirac's- in other words, the prediction of the positron does not uniquely prove the validity of the Dirac equation. As I recall, Weinberg then asserted that the correct formalism in which to write a relativistic treatment of the electron is not Dirac's but instead that of quantum field theory, and that Dirac was right for the wrong reasons.

Can anyone point me to the reference in which Weinberg originally made this argument?