Facts cannot be proved , one can only experience them

To those who are always denying spiritual reality by saying, ‘That cannot be proved; and everything must be proved,’ I should like to put one question: Would it have been possible to prove the existence of a whale in the physical world if none had ever been discovered? Nothing can be proved,unless it can be shown in the same way as a reality; even in the spiritual world one must experience that which is reality.

Source: Rudolf Steiner – GA 157a – The Forming of Destiny and Life after Death – Lecture 4 – Berlin, 7th December 1915