Plurality of Worlds. The Origins of the Extra-Terrestrial Life Debate from Democritus to Kant (review)

Book Reviews Steven J. Dick. Plurality of Worlds. The Origins of the Extra-Terrestrial Life Debate from Democritu~ to Kant. Cambridge University Press, 1982. ix, 246 pp. $34.5 o. Exobiology is a science in search of a subject. Outside planetary and lunar astronomy , where the case for extraterrestrial life has been largely surrendered on good empirical grounds, the empirical base for present-day exobiology is almost as exiguous as it was in the eighteenth century. The weight then given to natural theology has now shifted to biology, but some of the biology is hardly less speculative than the theology was. The author speaks of "the extra-terrestrial life debate [as] a case study of science attempting to function at its limits" 077)- At the limits, it is even more obvious than in the center that metaphysical paradigms and human expectations and wishes fill the vast gaps between observations, determine how the observations are to be interpreted, and even color the observation data themselves. There are only a few more facts pointing to the existence of extraterrestrial life than there were two centuries ago, so reading this fine book might induce in some exobiologists a healthy caution about their more confident and extravagant claims. However far we are from an answer to the question about extra-terrestrial life, the problem of the plurality of worlds can be judged to have been solved. Though the two questions mentioned in the title are often merged (occasionally even by the author), he rightly insists that answers to them are independent of each other. The late medieval critics of Aristotle's theory of the uniqueness of the world prepared the way for the readier acceptance of Lucretius when De rerum natura was finally recovered . The general shift from a plurality of logically possible but intrinsically unobservable worlds (mundi dissiti) to a plurality of observable worlds required a conceptual shift in the understanding of natural and violent motions, which had as corollary a multiplicity of centers of natural motions. The conceptual obstacle to plurality was that if there were more than one world, one and the same motion would be both violent and natural. Once this obstacle had been overcome by the conception of a multiplicity of centers of motion and the relativity of motion with respect to each, the problem of plurality became the empirical one of determining by observation which of the observable celestial bodies were centers of natural motions. Many were discovered; an infinity was hypothesized. The author, properly concentrating on the development of the quasi-empirical interpretation of the concept of other worlds (other solar and stellar systems within one universe) and the decline of the conception of mundi dissiti, overlooks the juxtaposition of these two concepts in Kant's early work. The Natural History and Theory of the Heavens teaches the plurality of worlds within a single Newtonian universe, but [365] 366 JOURNAL OF THE HISTORY OF PHILOSOPHY ~2:3 JULY 1984 the essay on Lebendige Kri~fte (sec. 8), which he does not mention, proposes the existence of worlds not part of the system and therefore intrinsically unknowable. Though the former writing has been influential in the history of science while the latter has been almost forgotten, the concept of unknowable worlds was more important in the development of Kant's own philosophy. Just as Aristotle's teaching of the uniqueness of the world was the main obstacle to the acceptance of plurality, scriptural teaching (at least ex silentio) of the uniqueness of man was the main obstacle in the way to belief in extraterrestrial life. But when good apologetic grounds were found for asserting the compatibility of extraterrestrial life with Christian monotheism, the question of the actual existence of extraterrestrial life was, unlike the question of plurality, still far from being answered by observation. The only genuinely empirical investigations then relevant to extraterrestrial life were limited to the moon; but in the absence of data and with confidence in plurality, fancy was encouraged by natural theology to speak of an infinity of inhabited worlds, Each stage in the process of'filling the sky with worlds and peopling the worlds with intelligent beings required pari passu some changes in...