Reason -- this likewise is unreliable and not to be depended upon. This human world is an ocean of varying opinions. If reason is the perfect standard and criterion of knowledge, why are opinions at variance and why do philosophers disagree so completely with each other? This is a clear proof that human reason is not to be relied upon as an infallible criterion. For instance, great discoveries and announcements of former centuries are continually upset and discarded by the wise men of today. Mathematicians, astronomers, chemical scientists continually disprove and reject the conclusions of the ancients; nothing is fixed, nothing final; everything is continually changing because human reason is progressing along new roads of investigation and arriving at new conclusions every day. In the future much that is announced and accepted as true now will be rejected and disproved. And so it will continue ad infinitum.

‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 21

Religion must be reasonable. If it does not square with reason, it is superstition and without foundation. It is like a mirage, which deceives man by leading him to think it is a body of water. God has endowed man with reason that he may perceive what is true. If we insist that such and such a subject is not to be reasoned out and tested according to the established logical modes of the intellect, what is the use of the reason which God has given man?

‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 63

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

Therefore, human reason, as already pointed out, is by its very nature finite and faulty in conclusions . . . Inasmuch as the source of traditions and interpretations is human reason, and human reason is faulty, how can we depend upon its findings for real knowledge?

‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 21