Arguments for or against SETI rarely invoke the theological and philosophical evolution behind such reasoning. Tipler, in his 1981 article, presents an historical approach to the concepts of extraterrestrial intelligence (ETI). The debates and arguments for or against ETI periodically reappear throughout history, albeit slightly refashioned to reflect the principles of any contemporaneous philosophical movement. Tipler introduces two related philosophies: the principle of plenitude and the plurality of worlds. The principle of plenitude surmises that what can exist must exist, such that the Universe contains all forms of existence and intelligent life must exist in elsewhere. The plurality of worlds describes the Universe as infinite, producing an indefinite number of worlds which may harbor life. Tipler then provides the historical evolution of these ideas discussing the Greco-Roman debates, the Copernican revolution, the scholasticism of the Medieval period, the Enlightened support for the plurality of worlds, and finally the scientific view of these philosophies.
Of particular interest was to see the evolution of the concept of the plurality of worlds. Prior to the Copernican revolution, the Ptolemaic Universe reigned supreme and the plurality of worlds meant an infinite number of self-contained Universes with a central Earth. Tipler mentions that despite having the physics wrong, this particular thought is akin to the modern support of ETI by the principle of mediocrity. The principle of mediocrity states that we are not special in the Universe such that, given the existence of life on Earth, life exists in other Earth-like planets in the Universe. The Christian argument against ETI was also intriguing. St. Augustine wrote that the uniqueness of Christ meant there was no other intelligent life, else they would have a separate Christ for such a world. St. Thomas Aquinas argued that God was perfect; which was inconsistent with the plurality of worlds as this would be an act in vain if similar worlds existed and an act of imperfection if dissimilar worlds existed. It was not until the nineteenth century that the plurality of worlds was used to argue for ETI and against Christianity, as seen by the excerpt from Thomas Paine’s Age of Reason.
The post-nineteenth century pervasiveness of the plurality of worlds should be viewed through a critical scientific lens. It was through the scientific understanding of the geological, biological, and chemical evolution of the Earth that the principle of plenitude was rejected. Tipler repeats the arguments of William Whewell and Alfred Wallace, which were aligned with that of evolutionary biologists in that they argued the Earth was uninhabited for most of its history and, to our knowledge, the conditions for life and natural selection are incompatible with harsh environments elsewhere. Tipler, unfortunately, does not muse over the fact that a post-Enlightened society would use the plurality of worlds to fuel the imagination and foment the idea that humans are not alone in the Universe (i.e. It Came From Outer Space or the spoof, It Came From Planet Earth). Most surprising to this blogger was that modern evolutionists argued against ETI while physicists and theologists appeared to have no issues with it. While the scientific community, with achievements such as the development of radio communication and the Miller experiment, may have cemented the belief in the plurality of worlds and the search for ETI, the larger biological and chemical considerations appeared to be ignored during the late twentieth century. Despite the support the principle of mediocrity provides ETI, it is imperative to address all scientific concerns regarding ETI. Perhaps it is no surprise that astrobiology has recently developed to as a response to the complex scientific nature of the search for ETI.