The Divine Institutes

It pleases me in this place to censure the folly of those who, while they wish to show that the senses are false, collect many instances in which the eyes are deceived; and among them this also, that all things appear double to the mad and intoxicated, as though the cause of that error were obscure. For it happens on this account, because there are two eyes. But hear how it happens. The sight of the eyes consists in the exertion of the soul. Therefore, since the mind, as has been above said, uses the eyes as windows, this happens not only to those who are intoxicated or mad, but even to those who are of sound mind, and sober. For if you place any object too near, it will appear double, for there is a certain interval and space in which the sight of the eyes meets together. Likewise, if you call the soul back as if to reflection, and relax the exertion of the mind, then the sight of each eye is drawn asunder, and they each begin to see separately.

If you, again, exert the mind and direct the eyesight, whatever appeared double unites into one. What wonder, therefore, if the mind, impaired by poison and the powerful influence of wine, cannot direct itself to seeing, as the feet cannot to walking when they are weak through the numbness of the sinews, or if the force of madness raging against the brain disunites the agreement of the eyes? Which is so true, that in the case of one-eyed men, if they become either mad or intoxicated, it can by no means happen that they see any object double. Wherefore, if the reason is evident why the eyes are deceived, it is clear that the senses are not false: for they either are not deceived if they are pure and sound; or if they are deceived, yet the mind is not deceived which recognises their error.