Persons such as Isaac Asimov and Carl Sagan say that tens of thousands (maybe tens of millions) of planets will fulfill the conditions for the suppor… - R. A. Lafferty

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Persons such as Isaac Asimov and Carl Sagan say that tens of thousands (maybe tens of millions) of planets will fulfill the conditions for the support of life. And then they take the rather deceptive step from the ‘possibility of life’ to the ‘inevitability of life’ by such connivance as would shame a crooked gambler. They posit towering numbers of ‘civilizations’ on those ‘possibility-of-life planets’, at least half of them to be more advanced than the Civilization of Earth and Humankind.
But there is a strong element of Advocacy Science in this. There is a great and powerful lobby advocating the existence of great numbers of superior civilizations. One reason for this is that the secular-liberal-agnostic-relativistic faction of scientists cannot allow the uniqueness of anything, not of Earth, not of Life, certainly not of Human Life, most certainly not of existing Human Civilization. To allow the uniqueness of any of these things, they would have to cease to be secular-liberal-agnostic-relativistic persons. And the shock of changing their style would kill all of them.
Science Fiction also has a vested interest in there being a multiplicity of inhabited worlds and civilizations. That is one of the small number of things that Science Fiction is about. But Science Fiction is, after all, only a fiction.