By your terms, NOTHING is possible unless it is shown.
Well, imagine it from the contrary perspective. How do you propose to tell the difference between the things that are possible but haven't happened yet, and the things that haven't yet happened because they're impossible and will therefore never happen?
You can make reference to a wider body of knowledge, say physics, and conclude that a certain thing is likely not possible
because it would mean that all our physics knowledge is wrong. But it's likely that our physics is
wrong, at least partially, so what does that prove?
Epistemological modesty means making the minimal, most defensible conclusions.
Edited by crashfrog, : Autocorrect error.