Niven, for example, is generally considered a writer of "hard science fiction.'' J. G. Ballard is not. Niven's stories [have] two-headed aliens, telepathic powers, various flavors of time-travel, galactic cataclysms, hyper-drives, tractor beams, and so forth. Most of Ballard's novels have been rather tight extrapolations of a world drastically altered by one reasonably plausible meteorological change, and even his later more stylistically dense works don't ask the reader to swallow… scientific improbabilities whole. Hal Clement's alien creatures are part of the hard science fiction canon, but Cordwainer Smith's Underpeople are not. Aficionados of hard science fiction accept Poul Anderson's medieval space cultures without a murmur but eschew the future worlds of Mack Reynolds which are worked out with a much more sophisticated and rigorous knowledge of economics and politics.