No mathematical proposition can be proven true by observation. It follows that it is impossible to know any mathematical proposition to be true. The conclusion follows logically if which one of the following is assumed? ['Knowing a proposition to be true is impossible only if it cannot be proven true by observation.', 'Observation alone cannot be used to prove the truth of any proposition.', 'If a proposition can be proven true by observation, then it can be known to be true.', 'Knowing a proposition to be true requires proving it true by observation.']