1. This listing is confined to those relevant materials which I have found particularly interesting or useful. It does not aspire to comprehensiveness. A much fuller bibliography is given in MacGowan and Ordway (1966).
2. Allen, T. (1965) The Quest: A Report on Extraterrestrial Life. Philadelphia: Chilton Books. (An imaginative survey of the issues.)
3. Anderson, P. (1963) Is There Life on Other Worlds? New York and London: Collier-Macmillan. (Chapter 8 “On the Nature and Origin of Science” affords many perceptive observations.)
4. Ball, J. (1973) “The Zoo Hypothesis,” Icarus, 19, pp. 347–349. (Aliens are absent because the Intergalactic Council has designated Earth a nature reserve.)
5. — (1980) “Extraterrestrial Intelligence: Where Is Everybody?”, American Scientist, 68, pp. 565–663.