Errors of Sight

Errors of our eyes concerning shapes

Our sight leads us less into error when it represents shapes to us than when it represents anything else to us; because shape in itself is nothing absolute, and its nature consists in the relationship that exists between the parts that bound some space and a point that we conceive within that space, and which we may call, as in the circle, the center of the figure. However, we are mistaken in a thousand ways concerning shapes, and we never know any of them through the senses with ultimate exactness.

We have no knowledge of the smallest bodies

We have just proven that our sight does not make us see every kind of extension, but only that which has a considerable relation to our body, and that for this reason we do not see all the parts of the smallest animals, nor those that compose all bodies, whether hard or liquid. Thus, being unable to perceive these parts because of their smallness, it follows that we cannot perceive their shapes, since the shape of bodies is only the boundary that limits them. Here then is already an almost infinite number of shapes—and even the greatest number—that our eyes do not reveal to us at all; and they even lead the mind, which trusts too much in their capacity and does not examine things enough, to believe that these shapes do not exist.

The knowledge we have of the largest is not exact

As for bodies proportionate to our sight, which are very few in number compared to the others, we roughly discern their shape, but we never know it exactly through the senses. We cannot even be assured by sight whether a circle and a square, which are the two simplest shapes, are not actually an ellipse and a parallelogram, even when these figures are in our hands and right before our eyes.

I say more: we cannot even distinguish exactly whether a line is straight or not, especially if it is somewhat long; for that we need a ruler. But what then? We do not know whether the ruler itself is as we suppose it ought to be, and we cannot be entirely certain of it. Yet without knowledge of the line, one can never know any shape, as everyone knows well enough.

This is what can be said in general about shapes that are right before our eyes and in our hands; but if we suppose them far away from us, how much change shall we find in the projection they make on the back of our eyes? I do not wish to stop here to describe them; they will easily be learned from some book on optics, or from examining the shapes found in paintings. For since painters are obliged to change almost all of them so that they appear natural, and to paint, for example, circles as ovals, this is an infallible sign of the errors of our sight regarding objects that are not painted. But these errors are corrected by new sensations, which one might perhaps regard as a kind of natural judgment and which one might call judgments of the senses.

Explanation of certain natural judgments that prevent us from being mistaken.

When we look at a cube, for example, it is certain that all the sides we see almost never make a projection or image of equal size on the back of our eyes, since the image of each of these sides that is painted on the retina or optic nerve is very similar to a cube painted in perspective, and consequently the sensation we have of it should represent the faces of the cube as unequal, since they are unequal in a cube in perspective. Yet we see them all as equal, and we are not mistaken.

Now, one might say that this happens by a kind of judgment that we make naturally, namely: that the faces of the cube that are farthest away and seen obliquely should not form images on the back of our eyes as large as the faces that are closer. But, since the senses only sense and never strictly judge, it is certain that this judgment is only a composite sensation, which consequently can sometimes be false.

However, what is in us merely sensation, being considered in relation to the author of nature who excites it in us as a kind of judgment, I sometimes speak of sensations as natural judgments, because this manner of speaking serves to give an account of things; as can be seen here, in the ninth chapter, toward the end, and in several other places.

These same judgments deceive us in particular encounters.

Although these judgments of which I speak serve to correct our senses in a thousand different ways, and that without them we would almost always be mistaken, they nonetheless do not fail to be occasions of error for us.

If it happens, for example, that we see the top of a bell tower behind a great wall or behind a mountain, it will appear to us quite close and quite small. If afterward we see it at the same distance, but with several stretches of land and several houses between us and it, it will undoubtedly appear to us more distant and larger; although in both ways the projection of the rays of the bell tower, or the image of the bell tower that is painted on the back of our eye, is exactly the same. Now, one might say that we see it larger because of a judgment that we make naturally, namely: that since there is so much land between us and the bell tower, it must be farther away, and consequently larger.

If, on the contrary, we see no land between our eyes and the bell tower, even though we know from other sources that there is much of it and that it is very far away—which is quite remarkable—it will nevertheless appear to us very close and very small, as I have just said. And one may also think that this happens by a kind of natural judgment in our soul, which thus sees this bell tower because it judges it to be five or six hundred paces away. For ordinarily our imagination does not represent more extension between objects unless it is aided by the visible sight of other objects that it sees in between, and beyond which it can still imagine.

This is why when the moon rises or sets, we see it much larger than when it is very high on the horizon; for when it is very high, we see no objects between it and us whose size we know in order to judge the size of the moon by comparison with them. But when it has just risen or is about to set, we see between it and us several stretches of countryside whose size we roughly know; and thus we judge it to be farther away, and because of that we see it as larger.

And it should be noted that when it is high above our heads, although we know very certainly by reason that it is at a very great distance, we nevertheless continue to see it as very close and very small; because in fact these natural judgments of sight are based only on perceptions of sight itself, and reason cannot correct them. So that they often lead us into error by making us form free judgments that agree perfectly with them. For when one judges as one senses, one is always mistaken; although one is never mistaken when one judges as one conceives, because the body instructs only for the body, and only God always teaches the truth, as I will show elsewhere.

These false judgments deceive us not only in the distance and size of bodies—which is not the subject of this chapter—but also by making us see their shape as other than it is. We see, for example, the sun and the moon and other very distant spherical bodies as if they were flat and like circles; because at that great distance we cannot distinguish whether the part facing us is closer to us than the others, and because of that we judge it to be at an equal distance. It is also for the same reason that we judge that all the stars and the blue that appears in the sky are at roughly the same distance as their neighbors and as if on a perfectly convex and elliptical vault, because our mind always supposes equality where it sees no inequality; yet it should positively recognize it only where it sees it with evidence.

We do not stop here to explain at greater length the errors of our sight concerning the shapes of bodies, because one can learn about them from some book on optics. This science, in fact, teaches only the way to deceive the eyes; and all its skill consists only in finding means to make us make the natural judgments of which I have just spoken at times when we should not make them. And this can be accomplished in so many different ways that of all the shapes in the world, there is not a single one that cannot be painted in a thousand ways; so that sight will infallibly be deceived by them. But this is not the place to explain these things thoroughly. What has been said suffices to show that we should not trust our eyes so much, even when they represent the shape of bodies to us; although in matters of shape they are much more faithful than in any other encounter.