People almost invariably arrive at their beliefs not on the basis of proof but on the basis of what they find attractive.

People are generally better persuaded by the reasons which they have themselves discovered than by those which have come into the mind of others.

Yet it is folly to argue against determined hardness; eloquence may strike the ear, and the language of sorrow draw forth the tear of compassion, but nothing can reach the heart that is steeled with prejudice.

...and when is enough proof enough?

Science replaces private prejudice with public, verifiable evidence.

Einer hat immer Unrecht: aber mit zweien beginnt die Wahrheit. Einer kann sich nicht beweisen: aber zweie kann man bereits nicht widerlegen.