A rebuttal of Tipler's Argument, was put forward by Carl Sagan and William Newman of Cornell University.1 They first pointed out that Tipler had underestimated the number of von Neumann probes there ought to be. With exponential growth, a single self-replicating probe could be expected to convert the entire mass of the Galaxy into copies of itself within 2 million years. Any species intelligent enough to build such a probe, Sagan and Newman argued, would also be intelligent enough to realize the danger of it and so would not embark upon the project in the first place. In the event of a von Neumann probe being released either accidentally or maliciously, it would be a prime duty of other, responsible civilizations, said Sagan and Newman, to stamp out the "infection" before it could spread.
1. Sagan, C., and Newman, W. "The Solipsist Approach to Extraterrestrial Intelligence," Quarterly Journal of the Royal Astronomical Society, 24, 113 (1983).