The second criterion is that of the intellect, which was the principal criterion of comprehension for those pillars of wisdom, the ancient philosophers. They deduced things through the power of the mind and relied on rational arguments: All their arguments are based upon reason. But despite this, they diverged greatly in their opinions. They would even change their own views: For twenty years they would deduce the existence of something through rational arguments, and then afterwards they would disprove the same, again through rational arguments. Even Plato at first proved through rational arguments the immobility of the earth and the movement of the sun, and then subsequently established, again through rational arguments, the centrality of the sun and the movement of the earth. Then the Ptolemaic theory became widespread, and Plato's theory was entirely forgotten until a modern astronomer revived it. Thus have the mathematicians disagreed among themselves, even though they all relied on rational arguments (83:3)

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