No ET! Ever?

[Anomalocaris]

Evolution recognizes that certain traits develop under certain conditions and other traits develop under other conditions. Suppose there were only one condition on Earth. Say the entire Earth was like it is in Alaska or the North pole. We could presuppose that life cannot exist without some way to protect itself against cold. If we never encounter any other conditions, our presupposition remains safe.

Actually, what we are doing is using deductions based on existing empirical knowledge to allocate limited research resource to maximize the chance of success, not assuming everything out there must conform to deductions based on existing empirical knowledge.

If we live on a plant where the whole environment is like the North Pole, then we have empirical knowledge that life is possible under North Pole like conditions.   Let's suppose we have no great confidence in our chemistry, so we can't say whether life is possible if it were 70 degrees warmer.   We have limited research resource, and we are just starting to look for alien life, how do we proceed?   Do we dissipate our limited resources on the infinite varieties of environment not known to be able to support life, or do we focus most of them on environment similar to ours, which we already know can support life?

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Except through sheer baseless fantasy, we have not yet been able to dream up, even at high level, how life can be possible without some chemical basis that would allow the formation of a very large variety of very complex, interacting molecules, and some medium that dissolve constituents of these molecules to transport them to a central location, without also instantly destroying these molecules once they form.

We may not yet know much of all there is to know about physics, but we think we have a good working knowledge of chemistry.  Chemistry say there is strong reason to suppose the rather unique chemicals and physical properties of water and carbon make these two by far the best candidates to fulfill the above stated conditions.

So it's not just because earth life is water and carbon based. It's no other chemical can come close to providing a similarly board basis for metabolism and genetic inheritance.

I'm not sure how to make your conditions for life independent from Earth conditions. Can we say no other model is possible under any conditions when we haven't seen any other model?

The difference between science and religion is the religionist thinks he knows everything and dismisses anything that falls outside his accept truth. But the scientist is open to whatever presents itself as valid fact, even if he hitherto though it impossible.

Chemistry is independent of earth conditions.  We make our deduction based on chemistry, not earth conditions.