A rugged, personal critique of science fiction past and present as Mr. Wollheim looks at the multitude of universes created by as many writers and the moral and philosophic imperatives which govern them. In the beginning there was Verne and Wells, gadgetry and concern; then came the inevitable stretch into a body of basic mechanistic cosmology which is still flourishing. There were also the early fourth dimensional pills of Ray Cummings; Hamilton's pioneering galactic civilization: the ""cosmic vision"" of Stapleton and Asimov's ""psychohistory,"" etc. Wollheim pays tribute to what is probably the best of the lot: Van Vogt, Anderson, Simic, Heinlein (unequivocally set down as ""the world's greatest living science fiction writer""). He takes account too of the new nihilism, reflecting very real earthbound problems; but deep in the heart of Mr. Wollheim's commitment to science fiction is its outward thrust toward possibility.