Book Review 6^ larly Dirac while writing the general rules for the quantum mechanics of any system incorporated the ideas of both matrix mechanics (Heisenberg) and wave mechanics (Schrodinger). What were considered to be incompatible concepts were shown to be equivalent by Dirac. Stephen Hawking observes: "Of the three founders of modern quantum mechanics, Heisenberg and Schrodinger can claim to have caught the first glimpses of the theory. But it was Dirac who put them together and revealed the whole picture." Such a matter of fact description of the evolution of quantum ideas does not capture the excitement, drama and the resistance to new ideas, even among those who were actively propounding them. The articles in this volume do give a flavour of the times. It was in 1929-30 that Dirac came to the conclusion of "unavoidable necessity" of anti-matter. The elusive particle with a positive charge which had to be associated with the electron as its anti-matter was considered to be proton for a while since no one dared to postulate an entirely new particle those days. Dirac noted later "I did not dare to postulate a new particle at that stage, because the whole climate of opinion at that time was against new particles". But by 1931, he had come to the conclusion that the new particle had to be postulated. "This would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge as the electron. We may call such a particle an anti-electron (positron later)." It required courage on the part of Dirac to pursue the theory of an undetected particle. At the same time when Murray Gell-Mann asked Dirac why he had not predicted the positron immediately, the characteristic reply of Dirac was "pure cowardice." Possibly in the creative processes of such intellectual dimensions, the courage of new idea finally does overtake the cowardice generated in the face of dominant opinion. Dirac himself provides an insight into his approach to creative ideas "... Schrodinger and I both had a very strong appreciation of mathematical beauty, and this appreciation of mathematical beauty dominated all our work. It was a sort of act of faith with us that any equations which describe the fundamental laws of Nature must have great mathematical beauty in them. It was like a religion with us. It was a very profitable religion to hold, and can be considered the basis of much of our success." Rationality of Dirac's arguments and brevity of his expression are perhaps best summed up in what was known as one of his principles in Cambridge: One must not start a sentence before one knows how to finish it. RAJENDRA PRASAD