The second criterion is that of the intellect. The ancient philosophers in particular considered the intellect to be the most important agency of judgment. Among the wise men of Greece, Rome, Persia and Egypt the criterion of true proof was reason. They held that every matter submitted to the reasoning faculty could be proved true or false and must be accepted or rejected accordingly. But in the estimation of the people of insight this criterion is likewise defective and unreliable, for these same philosophers who held to reason or intellect as the standard of human judgment have differed widely among themselves upon every subject of investigation. The statements of the Greek philosophers are contradictory to the conclusions of the Persian sages. Even among the Greek philosophers themselves there is continual variance and lack of agreement upon any given subject. Great difference of thought also prevailed between the wise men of Greece and Rome. Therefore, if the criterion of reason or intellect constituted a correct and infallible standard of judgment, those who tested and applied it should have arrived at the same conclusions. As they differ and are contradictory in conclusions, it is an evidence that the method and standard of test must have been faulty and insufficient.
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 254)