1. The zoo hypothesis;Ball;Icarus,1973
2. Model Theory for Modal Logic: Kripke Models for Modal Predicate Calculi;Bowen,1979
3. Logical Foundations of Probability;Carnap,1962
4. An explanation for the absence of extraterrestrials on Earth;Cox;Q.J.R. Astron. Soc.,1976
5. The search for extraterrestrial artifacts (SETA);Freitas;J. Brit. Interplanet. Soc.,1983