A proof can only be valid for two persons if they agree on the
same axioms. Axioms are either assumed truths or self-evident
truths. If axioms are assumed, then the proof does not say
anything about reality for the persons involved. If axioms are
self-evident then the proof is a conclusion about reality for
them. So a proof about reality is valid for two persons only if
the axioms are self-evident for both of them.