> Humans define what they want to call ``reality''.
> Who decide when there exists two conflicting definitions?
Reality decides what is real. If in your opinion of reality, a certain bridge is safe, you are likely to go ahead and use that bridge. And if you are wrong in your opinion of reality, you may end up plunging to your death.
Ultimately, evolution selects for people are good at perceiving reality correctly. Since the better you are at perceiving reality, the more likely you will survive.