The American cosmologist Frank Tipler out forward an argument in 19811 in opposition to the view that the Galaxy is populated by other intelligent races. It is based on the supposition that the most effective and economical way to explore the Galaxy is with autonomous self-replicating spacecraft known as von Neumann probes. The fact that no evidence of such probes has yet been found is proof, said Tipler, that we are alone. His argument was cited by Senator William Proxmire in his efforts, in the early 1980s, to halt funding for NASA's SETI program. Tipler's argument was effectively countered, however, by Carl Sagan and William Newman (see Sagan's Response).
1. Tipler, F. J. "Extraterrestrial Beings Do Not Exist," Quarterly Journal of the Royal Astronomical Society, 21, 267 (1981).