a fact is simply something we perceive to be true on our plane of existence.
you can’t say a rock is real, you could be in a simulation.
Whilst you cant disprove you are in a simulation, you have to assume that you aren’t since no evidence points to it.
If you never believe anything you will never do anything.
All facts are subjective, but we already have a subjective plane of existence. You must assume facts are actually fact.