In this brief document, Tipler reviews the history and development of the question of the plurality of worlds/extraterrestrial intelligence (ETI), from the earliest Greek thinkers through to the modern scientific era. This “meta” paper aims to justify the conclusion that the modern proliferation and acceptance of the ETI question is a consequence of developments in philosophical, theological, and empirical reasoning that been made throughout the centuries. He constructs a temporal chain linking advancements in thinking on the subject (supported by direct quotations of the primary sources), revised and modified with each new generation, that continues through to the current time. This paper is relevant to our discussion of SETI because it is one of few which attempt to comprehensively depict the status of thinking regarding the ETI question throughout history. In particular, the Greeks were separated into two schools, those who subscribed to plurality and plenitude, and those who objected; the Stoics, Pythagoreans, and Epicureans fell into the former camp, whereas the Platonists and Aristotelians the latter. Since the cosmology of Aristotle was highly influential on the thought of the medieval scholars, the early Church Doctors concluded the concept of plurality to be antithetical to Christian theology. Not until the time of the Copernican Revolution and the emergence of the Principle of Mediocrity did the idea that other worlds could be inhabited reemerge, and when it finally did, it did so with great force. One interesting feature of the history that this blogger did not as of yet know was the opinion held by the early Darwinists through to the modern evolutionary biologists, which turns out to be a pessimistic one on the grounds of the improbable series of millions of mutations which are requisite for the development of human-level intelligence. Tipler concludes the history at the end of the 20th century, when feelings regarding the ETI are positive due to advances in radio communication technology, theories of stellar and planetary formation and abiogenesis, and the contemporary cosmology which supported the thesis. Since this paper is not really a scientific paper in itself, but rather an exploration of the history of scientific and pre-scientific thought on the matter, it is fitting that a subsequent paper, to be written either presently or at some future point, should contain a continued account of the ETI question by way of a comprehensive treatment of SETI developments through the 21st century.
