Aristotle's Problems

Why does one thing appear as two? Based on the various disposition of the thing seen, we say that there are four causes on the part of the viewer why one thing appears as two. The first of these is: since the optic nerves, directed from the eyes to the anterior chamber of the brain, intersect each other in the manner of the lines of a triangle—the measure of the distance of which is that which exists between the two eyes—it happens sometimes that one or the other of the nerves is elevated or lowered from the place of contact. Then, that which it receives from the eyes, passing from the visual spirit to the common sense, happens to meet at some single point; and then, because the two eyes report the light twice, one thing appears as two. This happens whenever one of the eyes is pressed, as if half-torn out, by the placement of a finger. The second cause is that sometimes the visual spirit, which runs from the head into the eyes, is moved beyond the bounds of natural measure toward the right or even toward the left in the eye, because of some windiness enclosed in it or in the eye, or because of some non-natural heat agitating it. Therefore, it receives the form as if it were a form moved from the right to the left eye, and thus it apprehends everything it perceives as if distended into two parts joined laterally to itself. The third cause is in the opposite redundancy, when the spirit is moved inward and outward: inward in respect to the common sense, and outward toward the crystalline lens of the eye so that you may perceive forms. For then it continually returns and receives; and before the return is completed, the reception begins while the sensible object remains present, and then one thing appears as two. This accident occurs in the spirit sometimes from an abundance of hot humor, as in drunkards, because the heat of the wine moves in opposite directions and the humor multiplies the flow; and there are very many accidents related to this one. For when someone sails, because he receives one part and is changed by another before he returns it to the common sense, he joins the same accidents together by the velocity of the movement of the spirit, so that all things seem to him to move similarly. Or if the end of a rod is whirled around, being flexible, and its position is perceived in one "where" before it is returned, it is perceived in another and a third; and thus the variation of its position is continually perceived and returned, and it seems to describe the base of a pyramid. This is the cause why a falling drop of water appears to be a continuous straight line, and seems like a point continually flowing. From this it also happens that by windiness or some other cause producing vertigo in the anterior part of the head, all things seem to be moved; and one falls toward the part to which one holds oneself, as if the other part were falling. The fourth cause is the squeezing of the perforated tunic in it: for when that is pressed inward and yields inward, the opening is then narrowed, and the thing appears smaller than it is; sometimes it projects further inward, and then the opening is dilated, and the thing appears larger than it is. On the part of the visible object, however, one thing appears as two subjects in which it exists according to its different being, just as in a mirror.

they are brought forth equally to a place, but not to indistinguishable places and bodies; and thus each eye, looking at its own, effects a double vision. That this is most true is certainly evident from the fact that by pressing one eye with a finger and propelling it upward, we seem to behold all things doubled.

Why do those who are cross-eyed by nature not see double? Because they do not have a perversion of the eyes upward or downward, but to the right and to the left; and again, the eyes having obtained the same position, they have, I say, their own rays in a straight line through the same position falling upon one body; and thus they report one color and one quality in the objects set before them by taking them up again.

Why do boys at puberty reach the "goat-scent" stage [hircissant] around the fourteenth year? Because human nature then, by dissociating the bodies, effects a great and sudden change of age; and the effect itself gives credit to the things we have said. For we may observe the breasts of a female increase because of the gathering of milk; likewise the regions around the hips because of the expectation of offspring; but the chest and shoulders of males because of the carrying of heavy weights imposed upon them. The testicles moreover for the reception and retention of the female; the private parts furthermore for easy access to females. The whole body is also unfolded to them, just as the changes and resolutions of diseases produced around the fourteenth year indicate. Moreover, the harshness of the voice, which is called hoarseness, causes the "goat-scent" [hircire]; the metaphor being taken from goats emitting their voice in such a way. For during the unfolding, it happens that the windpipe is harsh from the beginning, some parts of it being drawn apart more, others less. This is because the body of the throat itself is unequal along its interior surface; the spirit, therefore, as it exits through these parts, is as if cut short because of the inequality of the path, and it produces a voice that is unequal, harsh, and hoarse. Those occupied by phlegm also suffer this in the windpipe...