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All the analogies of nature lead us to believe that, whatever the process which led to life upon this earth—whether a special act of creative power or a gradual course of development—through that same process does life begin in every part of the universe fitted to sustain it. —Astronomer Simon Newcomb, 1905! Believers argue that life is likely to emerge wherever physical and chemi- cal conditions allow, making it a widespread phenomenon in the universe. Deniers claim that the origin of life is exceedingly rare, perhaps unique to the Earth. How can we address this complicated issue? Let’s begin with philosophical viewpoints about the origin of life, as described by Paul Davies. First, it was a miracle; this traditional view would be challenged by the discovery of an independent origin of life, or by the creation of life in a laboratory. Second, it was an accident, a highly improb- able event; finding an independent origin of life would challenge this view as well. Third, it was a natural process of high probability. Davies laid out principles underlying the case for extraterrestrial life. First is the Principle of Uniformity of nature: As the laws of nature are the same throughout the universe, the physical processes that produced life on Earth can produce life elsewhere. Second is the Principle of Plenitude: that which is possible in nature tends to become realized; if there is no impe- diment to the formation of life, life will form. Third is the Copernican Principle: The Earth does not occupy a special position in the universe, but apparently is a typical planet orbiting around a typical star in a typical galaxy. Each of those principles has been challenged by skeptics. Many biolo- gists believe that the origin of life was highly contingent, some putting the factor f(/) close to zero. They argue that the evolution of life as we know it is the result of such an extremely unlikely combination of circumstances shat Dash wae Le ee Ae Le 68 Probabilities: Life Miracle, Rare Accident, or Probable Event? that Earth may be its only home.