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| The art of M.C. Escher attempts to portray logical and physical impossibilities. |
When it comes to the existence of intelligent life outside of earth in the universe, I'm more persuaded by rare earth theory than SETI-Sagan ideas. See here, for example.
In dialogue with a friend (J.S.) I mentioned that, even if our universe is life-permitting, there's neither physical nor logical necessity re. the formation of life, and thus there's neither physical nor logical necessity that life would form outside of earth.
Obviously, life has formed on earth. Has it formed elsewhere in the universe? Ward and Brownlee (et. al.) have said: most probably there are other Earthlike planets on the universe where life has formed but, also most probably, no greater than a flatworm. There's is a probableistic theory, and inductive theory. Therefore it is not deductively certain that any life, outside of Earth, has formed. The formation of extra-Earth life is not recessary (i.e., not deductively so).
The formation of life outside of Earth is not physically necessary, given Earthlike conditions. The formation of life on Earth is not physically necessary. One could think of many reasons why, even given a life-friendly planet, life may not have formed. In fact, no physical conditions lead to necessary consequences, though some physical conditions lead to highly probable or improbable consequences.
The formation of life on Earth and outside of Earth is not logically necessary. The statement Physical life outside of earth does not exist does not form a logical contradiction, while statements like Object A is a square circle and John is a married bachelor do.
For more on physical possibility and physical impossibility, and logical possibility and logical impossibility, see Lewis Vaughn, The Power of Critical Thinking: Effective Reasoning About Ordinary and Extraordinary Claims. (This is the text I use in my mCCCC Logic classes.)
One more thing. To appeal to the immense size of our finite universe as a reason to conclude "There must be intelligent life elsewhere in the universe" is to fall into Saganistic Drake-equation thinking, and to miss the point of the probableistic Rare Earth Equation. For example, no matter how large our universe is no square circle exists in it, since square circles cannot exist. Using Ward and Brownlee, no matter how large our universe exists intelligent life probably does not exist, on the Rare Earth Equation.
