For example, I often hear that life could not develop in a universe where the fundamental constants were even slightly changed, or where certain physical laws were different. But if we're dealing with an entirely different universe with different "rules", then why does life have to continue to follow "our" rules? Once you start making new rules / laws of physics and changing things around, then it appears that all bets are off and there's nothing stopping you from changing something else, including how life itself works. Why is it still meaningful to discuss what is "impossible" in alternate universes like these?
This is different from asking questions like "what if 2 + 2 = 5?", which obviously doesn't make sense because it violates our inherent reasoning about mathematics and logic itself, and thus would be impossible in any universe, not just ours. But a statement such as "life exists and the speed of light is only 10 miles per hour" is, on the surface, something that could hypothetically exist in some alternate universe by the very fact that I can make this statement and imagine it as being so, even though it obviously is not true of our reality.
Are there examples of logical impossibilities that, on the surface, seem like they could be possible in another universe because we can "imagine" it as being so, but in fact would violate logic in a non-obvious way? (I suppose one example might be the Halting Problem, which is a logical impossibility that wasn't discovered until Alan Turing's proof, and, until one reads and understands the proof for it, sounds perfectly reasonable to imagine in some alternate reality.)