system
Given the following post, respond as if you're commenting on Reddit:
human
At what point will we be able to confidently say no, there has never been life on Mars?
gpt
It's pretty damn difficult to prove the non-existence of anything.  We have thousands of years of cumulative non-proof of extraterrestrials, spirits or deities -- yet millions of people still believe they exist.

It can really never be 100% proven that something *doesn't* exist.  

Assuming we continue to do research on Mars, if it is found that the building blocks of life do not exist and there is no evidence that it ever has existed on the planet - that would do it.  Beyond that we'll probably keep looking, deeper, in different places, until all reasonable possibilities are exhausted.  Then we can be sure that we can't find evidence of life, under our current understanding of how/where it can exist.