One is caused by natural vanity, for many men are so presumptuous that they believe they know everything, and, owing to this, they assert things to be facts which are not facts.  Tullius especially execrates this vice in the first chapter of the Offices, and St. Thomas in his book against the Gentiles, saying:  “There are many men, so presumptuous in their conceit, who believe that they can compass all things with their intellect, deeming all that appears to them to be true, and count as false that which does not appear to them.”  Hence it arises that they never attain to any knowledge; believing themselves to be sufficiently learned, they never inquire, they never listen; they desire to be inquired of, and when a question is put, bad enough is their reply.  Of those men Solomon speaks in Proverbs:  “Seest thou a man that is hasty in his words? there is more hope of a fool than of him.”

Another infirmity of mind is caused by natural weakness or smallness, for many men are so vilely obstinate or stubborn that they cannot believe that it is possible either for them or for others to know things; and such men as these never of themselves seek knowledge, nor ever reason; for what other men say, they care not at all.  And against these men Aristotle speaks in the first book of the Ethics, declaring those men to be insufficient or unsatisfactory hearers of Moral Philosophy.  Those men always live, like beasts, a life of grossness, the despair of all learning.

The third infirmity of mind is caused by the levity of nature; for many men are of such light fancy that in all their arguments they go astray, and even when they make a syllogism and have concluded, from that conclusion they fly off into another, and it seems to them most subtle argument.  They start not from any true beginning, and truly they see nothing true in their imagination.  Of those men the Philosopher says that it is not right to trouble about them, or to have business with them, saying, in the first book of Physics, that against him who denies the first postulate it is not right to dispute.  And of such men as these are many idiots, who may not know their A B C, and who would wish to dispute in Geometry, in Astrology, and in the Science of Physics.

Also through sickness or defect of body, it is possible for the Mind to be unsound or sick; even as through some primal defect at birth, as with those who are born fools, or through alteration in the brain, as with the madmen.  And of this mental infirmity the Law speaks when it says:  “In him who makes a Will or Testament, at the time when he makes the Will or Testament, health of mind, not health of body, is required.”

But to those intellects which from sickness of mind or body are not infirm, but are free, diligent, and whole in the light of Truth, I say it must be evident that the opinion of the people, which has been stated above, is vain, that is, without any value whatever, worthless.