I think the answer to the question here is to just admit that we don't know, and that we should be comfortable with accepting that fact, and everything it implies.
I personally think that life is about defining what matters to you. We all are "thinking machines" with their own construction and capabilities, and it is up to us to decide what we want, what we believe, and what we will do.
Both people are right, both people in your example who believe what they think have seen evidence that registers as fully valid and reasonable. Neither are incorrect, neither have flawed reasoning, they are just different people.
But, at the end of the day, neither really knows. Nobody does, not the real objective truth that applies to everyone else well, and sometimes it's best to have views that aren't objectively true, it betters society in the long run.