However the "law of biogenesis" teaches that life can only arise from existing life.
Actually, there is no "law of biogenesis."
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Is there a chance that God exists?
I suppose that there is. But to answer this question, we must look at the evidence that exists, and decide whether there is evidence in favor of the existence of a god, how good that evidence is, whether there is evidence against the existence of a god, and how good that evidence is.
Me, I think it is very unlikely that god exists.
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Can we assign a probability to the existence of God?
Not without more information. Probabilies are a form of mathematical model, and mathematical models of the real world are based on emprirical evidence. I don't know of any empirical studies that allow us to assign probabilities to the existence of a god.
The closest thing I can think of is to examine a large number of universes with life, and compare the number of these universes in which there is no god with the number in which there is a god.
Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.
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M. Alan Kazlev