(Previous Page) Part 5: SETI, Fermi and the Anthropic Principle     

Principle of Plenitude

     The Principle of Plenitude has existed since Aristotle's time, and it is still an important part of the extraterrestrial life debate today. Although our knowledge of how the universe may have began (the Big Bang), how many stars may exist, and how many planets may exist around those stars, we are no closer to answering the question: does life exist on those other planets?  It is another example of how technology and scientific knowledge do not necessarily bring us closer to settling the debate. 

     The most common definition of the Principle of Plenitude is that if the universe is to be as perfect as possible, it must be as full as possible. Another version of the principle refers to probabilities of events rather than kinds of objects as implied above. It says that there can be no possibilities that remain possibilities but are unrealized throughout eternity. More plainly this version states: given enough chances for the event to take place, it will take place given a long enough time span. In relation to extraterrestrial life, the Principle of Plenitude has more relation to the latter version in meaning: the universe is so big and so full that there must be other “Earths” out there. In light of the recent studies in exobiology, the idea has arisen that if life is capable of originating on a planet somewhere, it eventually must, according to the principle. 

     According to Arthur Lovejoy in Great Chain, "...no genuine potentiality of being can remain unfulfilled, that the extent and the abundance of the creation must be as great as the possibility of existence and commensurate with the productive capacity of a 'perfect' and inexhaustible Source, and that the world is better, the more things it contains."[1] Expanding Lovejoy's idea about the Earth out to the universe and we get the Principle of Plenitude as defined in this course. This is due to the infinity of the universe, in space and in time, so theoretically anything that possibly can happen, eventually will.

     Giordano Bruno, an Italian philosopher during the late 16th century, was an early supporter of an infinite universe. He also favored the Principle of Plenitude when he wrote On the Infinite Universe and Worlds, "Tenthly, since it is well that this world doth exist, no less good is the existence of each of the infinity of other words."[2] Many other prominent figures in the extraterrestrial life debate approved of the Principle of Plenitude, including Blaise Pascal, Fontenelle, Thomas Wright, Immanuel Kant, and Thomas Paine among others.

     Kant, described as the most creative philosopher of modern times, stated in Universal Natural History, "...the cosmic space will be enlivened by worlds without number and without end."[3] Immanuel Kant was one of the most vigorous advocates of the Principle of Plenitude during the late 1700's.Thomas Paine, during the late eighteenth century, launched one of the most vicous attacks against Christianity in his book Age of Reason.[4] Included in Paine's book were arguments about extraterrestrials and the idea of the Principle of Plenitude. He wrote, "Since, then, no part of our earth is left unoccupied, why is it to be supposed that the immenstity of space is a naked void, lying in eternal waste? There is room for millions of worlds as large or larger than ours, and each of them millions of miles apart from each other."[5]  Richard Dawkins, a figure from the modern debate, stated in The Blind Watchmaker, "Given infinite time, or infinite opportunities, anything is possible."[6] and later wrote, "How very conceited to assume that, out of all the billions of billions or planets in the universe, our own little backwater of a world, in our own little backwater of a solar system, in our own local backwater of a galaxy, should have been singled out for life?"[7] The latter statement by Dawkins really summarizes the tone of the Principle of Plenitude.

     The idea is also seen in Roy Mash's argument, Big Numbers and Induction in the Case for Extraterrestrial Intelligence.  The Lucretian argument deals with an infinite number of stars, galaxies, planets, universe, and so on.  It says that in an infinite universe with finite materials, the probability of an event being replicated is likely to happen. However, most of the assumptions used in this argument are not necessarily valid because they cannot be proven.  The Principle of Plenitude applies by saying that Earth is just a mediocre planet in an average universe and that we are not occupying a special place in the universe due the infinite number of planets, stars, etc.  Therefore we would conclude that if life could exist here, it could exist anywhere.

     He talks about a warehouse filled with boxes, millions or billions or more. He then says that suppose you walk into the warehouse and open just one box and find a single black marble inside it. Does this give you reason to believe that every other box contains a black marble? Is this the only one with the marble and it was possibly placed there by accident? His fictional characters then begin to discuss whether increasing the number of boxes in the warehouse makes the appearance of another identical box containing a marble more or less likely. Then what if the marble had an imperfection such as a gouge? This would seem to make it even less likely that the marbles in other boxes, if indeed there are, also have a gouge. It seems the only answer to the question would be to continue opening boxes one after another and see for yourself what, if anything, the boxes contain.  This situation applies to the universe, and the earth as the box with the black marble.  We do not know if there are life forms on other planets until we have explored this possibility for every planet that exists.

     It is important to realize that this particular argument for plurality plays off of big numbers and chance events that we, as humans, may not be able to fully comprehend. The number of stars in the universe is merely an estimate, however the figure representing the number of planets revolving around those stars does not represent a real figure, but rather an entity that is extremely large; our brains cannot even fathom numbers so big. Events occuring at, perhaps, once per aeon or once per every 1,000 million million millionth planet seems to us to rare as to be realistically impossible. This logic is unsound, however, due to the constraints of the development of the human brain.

     Richard Dawkins, an evolutionary biologist and popular science author, reminds us of the evolutionary journey of the human brain. He spoke of large numbers, like the ones describing the vastness of the universe, he wrote, "Nobody can really comprehend or imagine such a large number, and we just think of this degree of improbability as synonymous with impossible. But although we can't comprehend these levels of improbability in our minds, we shouldn't just run away from them in terror."[8] Dawkins pointed out that our brains were not developed, evolutionarily, in such a way to be able to "get" these kinds of numbers, or probabilities. He summed up his argument by stating quite bluntly, "But 'we', remember, are beings wose brains are equipped with a spotlight of comprehensible risk that is a pencil-thin bean illuminating the far left-hand end of the mathematical continuum of calculable risks. Our subjective judgement of what seems like a good bet is irrelevant to what is actually a good bet."[9]

     It is clear that the history of the extraterrestrial life debate has been littered with ideas that contain or were founded upon the Principle of Plenitude. Indeed many of more compelling, arguably more "likely" theories are based upon the notion that there are so many other planets in the universe that at least one of them has to contain some form of complex life.The Principle of Plenitude, coupled with other theories found in Webb's book Where is Everybody? creates an impressive argument for the existence of extraterrestrial life.

[1]  Michael J. Crowe, ed., the Extraterrestrial Life Debate, Antiquity to 1915: a Sourcebook, 2004, p. 36

[2] Giordano Bruno, On the Infinitet Universe and Worlds, pp. 38-44 in Michael J. Crowe, ed., the Extraterrestrial Life Debate, Antiquity to 1915: a Sourcebook, 2004, p. 40

[3] Michael J. Crowe, ed., the Extraterrestrial Life Debate, Antiquity to 1915: a Sourcebook, 2004, p. 139

[4] Michael J. Crowe, ed., the Extraterrestrial Life Debate, Antiquity to 1915: a Sourcebook, 2004, p. 221

[5] Thomas Paine, The Age of Reason, pp. 221-229 in Michael J. Crowe, ed., the Extraterrestrial Life Debate, Antiquity to 1915: a Sourcebook, 2004, p. 226 

[6] Richard Dawkins, Origins and Miracles in The Blind Watchmaker, JINS Extraterrestrial Life Reader, 2008, p. 93

[7] Richard Dawkins, Origins and Miracles in The Blind Watchmaker, JINS Extraterrestrial Life Reader, 2008, p. 95 

[8] Richard Dawkins, Origins and Miracles in The Blind Watchmaker, JINS Extraterrestrial Life Reader, 2008, p. 95

[9] Richard Dawkins, Origins and Miracles in The Blind Watchmaker, JINS Extraterrestrial Life Reader, 2008, p. 105

(Next Page) Part 7: The Goldilocks Enigma