The second is the method of reason, which was that of the ancient philosophers, the pillars of wisdom; this is the method of the understanding. They proved things by reason and held firmly to logical proofs; all their arguments are arguments of reason. Notwithstanding this, they differed greatly, and their opinions were contradictory. They even changed their views-- that is to say, during twenty years they would prove the existence of a thing by logical arguments, and afterward they would deny it by logical arguments-- so much so that Plato at first logically proved the immobility of the earth and the movement of the sun; later by logical arguments he proved that the sun was the stationary center, and that the earth was moving. Afterward the Ptolemaic theory was spread abroad, and the idea of Plato was entirely forgotten, until at last a new observer again called it to life. Thus all the mathematicians disagreed, although they relied upon arguments of reason. In the same way, by logical arguments, they would prove a problem at a certain time, then afterward by arguments of the same nature they would deny it. So one of the philosophers would firmly uphold a theory for a time with strong arguments and proofs to support it, which afterward he would retract and contradict by arguments of reason. Therefore, it is evident that the method of reason is not perfect, for the differences of the ancient philosophers, the want of stability and the variations of their opinions, prove this. For if it were perfect, all ought to be united in their ideas and agreed in their opinions (297:3)

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