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THE MORE WE LEARN about symmetry operations—space inversion, time reversal and particle‐antiparticle conjugation—the less we seem to understand them. At present, although still very little is known about the true nature of these discrete symmetries, we have, unfortunately, already reached the unhappy state of having lost most of our previous understanding. Let us, therefore, review the gradual evolution of our past concepts of these discrete symmetry operations.
As we expand our observation, we extend our concepts. Thus the simple symmetries that once seemed self‐evident are no longer taken for granted. Out of studies of different kinds of interactions we are learning that symmetry in nature is some complex mixture of changing plus into minus, running time backward and turning things inside out.