(1) As is demonstrated by Dick and Crowe, there is a long history of philosophical discussion of extraterrestrials. The early atomists were optimistic about extraterrestrials. They proposed an infinitely large universe, consisting solely of "atoms and the void", with different arrangements of atoms producing the diversity of objects we see around us. On this view, our planet and the organisms inhabiting it were created by chance collisions of atoms. Given this metaphysics, we can expect that other worlds with other living creatures will have arisen elsewhere. Lucretius, for example, says (quote taken from Kukla):
it is in the highest degree unlikely that this earth and sky is the only one to have been created … This follows from the fact that our world has been made by nature through the spontaneous and causal collision and the multifarious, accidental, random and purposeless congregation and coalescence of atoms whose suddenly formed combinations could serve on each occasion as the starting-point of substantial fabrics – earth and sea and sky and the races of living creatures.Following the Copernican revolution, the scholarly discussion of extraterrestrial life snowballed, and optimism was the dominant position. There were two primary justifications for belief in extraterrestrials. First, most scientists accepted the Copernican Principle, which claimed, to put it simply, that there is nothing special about the Earth (versions of this principle are still accepted today). The stars are suns just like our own; and all areas of the universe are subject to the same laws of nature. Thus it was expected that planets similar to the Earth would be prevalent throughout the universe. Studies of other solar system bodies seemed to confirm the similarity to the Earth: astronomers observed clouds on Jupiter and mountains on the Moon. Second, abundance of extraterrestrial life was held to follow from religious commitments. God would not be wasteful; he would not have created a vast universe filled with stars and planets for no reason. Obviously such other worlds were not created for the benefit of humankind, so their purpose must be to house other intelligent species. Many scholars, such as William Herschel, went as far as to suggest that the Moon, the comets, Saturn's rings, and even the interior of the Sun, were inhabited.
Against this optimism were those who held that extraterrestrial life, even if it does not explicitly contradict anything in the Bible, is at least in some tension with a religion that is structured around humanity's unique experience and history. Should we suppose that Jesus died and was resurrected on an infinite number of planets? This seems absurd. Or perhaps extraterrestrials have not fallen and so are in no need of redemption? But now we are supposing that the Earth is uniquely evil. Along with these religious concerns, by the mid-1800s scholars such as William Whewell were emphasizing scientific studies showing significant differences between the planets, such as spectroscopic studies of the Moon that revealed its lack of appreciable atmosphere. Philosophers engaged in a great debate about extraterrestrials throughout the 19th century.
(2) The scientific study of extraterrestrial life, which is part of the field of astrobiology, is a highly interdisciplinary field currently undergoing rapid progress, however its conceptual foundations have yet to be fully developed. For exmaple, what is the status of principles such as the anthropic principle and the aforementioned Copernican principle, and how should they be applied? How much can we learn from the Drake equation? So far, much of this work has been left to the scientists themselves. Philosophical study of extraterrestrials will allow philosophers to work closely with an increasingly powerful science, and on a topic that is of great interest to many people even outside of academia.
(3) The problem of extraterrestrials is connected with a variety of topics in philosophy of biology, philosophy of language, philosophy of mind, ethics, and so on. For example:
-- One of the first questions that arises in the search for extraterrestrial life is, what exactly are we looking for? What is life? This is, of course, a central problem in philosophy of biology.
-- Assuming that life does exist on other planets, how similar should we expect it to be to life on this planet? There has been a great deal of debate in philosophy of biology about the degree to which the evolutionary process is contingent, or is instead driven by trends that tend to produce convergence on the same solutions. A similar question is, does civilization require a humanoid form? Dolphins, octopi, and crows are intelligent animals, but it's difficult to see how they could build a civilization due to the limits of their morphologies - they simply cannot manipulate tools in the precise ways that humans hands can.
-- A number of authors have suggested that SETI is in some sense unscientific. To take Popper's criterion of falsifiability, for example, it seems that SETI research is organized around a proposition that is unfalsifiable, namely: there exist detectable extraterrestrial civilizations. SETI may therefore provide a good case study for discussion of the demarcation problem of how to distinguish science from non-science.
-- Putting aside the practical problem of the vast distances, is it possible even in principle to communicate with extraterrestrials? Translating human languages, such as Egyptian hieroglyphics, is often hard enough; the difficulties facing any interpreters of extraterrestrial signals will be orders of magnitude greater. Is there anything that could serve as a "universal langauge" - mathematics, for example? (I discuss the problem of extraterrestrial communication in this video.
-- Active SETI is a branch of SETI that attempts to contact extraterrestrials directly by sending messages into space. Various authors have argued that while listening for extraterrestrials is acceptable, we should refrain from sending messages ourselves because it poses potentially catastrhopic risks. We don't know the character or the capabilities of the civilizations we are attempting to contant. For all we know, they may well be able to do us great harm. (I discuss these risks in this video.
-- What will be the impact on our culture of discovering extraterrestrial life? Are contemporary religions such as Christianity compatible with the existence of extraterrestrials?
-- We may well discover simple microbial life elsewhere in the solar system; Mars and Europa, for example, remain possible abodes of life. What moral obligations would we have to such life? What kind of planetary protection policies should we adopt? How can we extend contemporary theories in environmental ethics to cover extraterrestrial life?
Why did philosophers lose interest in extraterrestrials? I don't have the historical competence to answer this definitively, but it's worth noting a few points. By the early 1900s, the scientific justification for belief in extensive extraterrestrial life had collapsed. Closer studies of the other planets of the solar system revealed them to be barren, desolate worlds, wholly unsuitable for life. The "canals of Mars" controversy had brought disrepute to the study of extraterrestrials. Furthermore, by this time the dominant view of the origin of the solar system was the Chamberlin-Moulton encounter hypothesis, which proposed that the solar system formed when another star passed close to the Sun and gravitational interaction drew out huge filaments of material that coalesced to form the planets. Planets were therefore likely to be extremely rare. Whereas the Copernican revolution had initially seemed to provide a scientific basis for optimism, extraterrestrials now once again entered the realm of mere speculation.
At around the same time, philosophy became dominated by the logical positivists, who held that philosophy cannot make empirical claims and instead focused on technical problems of meaning of scientific terms, theory reduction, the nature of scientific confirmation, etc. They saw little value in philosophical speculation on grand topics such as extraterrestrials. In later decades, into the 1940s and beyond, outlandish reports of UFOs and alien abductions have sullied the topic of extraterrestrials still further in the eyes of serious philosophers.
Whatever the reasons for the absence of philosophical study of extraterrestrials, I hope, in light of the great scientific progress on the problem and the wide range of philosophical issues it connects with, that more philosophers turn their attention to it in the near future.
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