  This was a most remarkable passage in retrospect. The symmetry that  he mentioned here, of physical laws with respect to the interchange  of right and left, had been introduced by Weyl and Wigner  independently into quantum physics. It was called parity  conservation, denoted by the symbol \(P\). The symmetry between  the past and future was something that was not well understood in  1930. It was understood later by Wigner, was called time reversal  invariance, and was denoted by the symbol \(T\). The symmetry with respect to positive and negative electricity was later called charge  conjugation invariance \(C\). It is a symmetry of physical laws  when you change positive and negative signs of electricity. Nobody,  to my knowledge, absolutely nobody in the year 1930, was in any way  suspecting that these symmetries were related in any manner. I will  come back to this matter later. What had prompted Weyl in 1930 to  write the above passage is a great mystery to me.