Although, of all our senses, touch is the one most constantly used, still, as I have said, its conclusions are the most rude and imperfect. This is because it is always used at the same time with sight; and because the eye attains its object sooner than the hand; the mind nearly always decides without appealing to touch. On the other hand, the decisions of touch, just because they are so limited in their range, are the most accurate. For as they extend no farther than our arm's length, they correct the errors of other senses, which deal with distant objects, and scarcely grasp these objects at all, whereas all that the touch perceives it perceives thoroughly. Besides, if to nerve-force we add muscular action, we form a simultaneous impression, and judge of weight and solidity as well as of temperature, size, and shape. Thus touch, which of all our senses best informs us concerning impressions made upon us by external things, is the one oftenest used, and gives us most directly the knowledge necessary to our preservation.
The sense of touch confines its operations to a very narrow sphere around us, but those of sight extend far beyond; this sense is therefore liable to be mistaken. With a single glance a man takes in half his own horizon, and in these myriad impressions, and judgments resulting from them, how is it credible that there should be no mistakes? Sight, therefore, is the most defective of all our senses, precisely because it is most far-reaching, and because its operations, by far preceding all others, are too immediate and too vast to receive correction from them. Besides, the very illusions of perspective are needed to make us understand extension, and to help us in comparing its parts. If there were no false appearances, we could see nothing at a distance; if there were no gradations in size, we could form no estimate of distance, or rather there would be no distance at all. If of two trees the one a hundred paces away seemed as large and distinct as the other, ten paces distant, we should place them side by side. If we saw all objects in their true dimensions, we should see no space whatever; everything would appear to be directly beneath our eye.
For judging of the size and distance of objects, sight has only one measure, and that is the angle they form with our eye. As this is the simple effect of a compound cause, the judgment we form from it leaves each particular case undecided or is necessarily imperfect. For how can I by the sight alone tell whether the angle which makes one object appear smaller than another is caused by the really lesser magnitude of the object or by its greater distance from me?