is that we only have one sample of life to work and make judgements from. All life that we can see seems to be built on one set of basic principles. And it looks to us like you need water, carbon, a set temperature range, some form of atmosphere (even if disolved in a liquid base) and some protection from extreme radiation.
But remember, that is based only on a sample of one.
Is there the possibility of life based on a whole different set of processes?
Certainly.
Would we recognize it if we ran into such a thing?
I don't know.
Aslan is not a Tame Lion