  Because the only fundamental particles that were known to exist at  that time were the electron and the proton, one was justifiably  reluctant to postulate the existence of new particles that had not  yet been observed experimentally; consequently, it was suggested that the positive electron should be identified with the proton. However,  Weyl was quite concerned about the identification of the proton with  the anti-electron. In the preface to the second German edition of his book Gruppentheorie und Quantenmechanik, Weyl (1928, 2 edn,  1931, VII) wrote 