To exercise the senses is not merely to use them, but to learn how to judge correctly by means of them; we may say, to learn how to feel. For we cannot feel, or hear, or see, otherwise than as we have been taught.
There is a kind of exercise, purely natural and mechanical, that renders the body robust without injuring the mind. Of this description are swimming, running, leaping, spinning tops, and throwing stones. All these are well enough; but have we nothing but arms and legs? Have we not eyes and ears as well? and are they of no use while the others are employed? Use, then, not only your bodily strength, but all the senses which direct it. Make as much of each as possible, and verify the impressions of one by those of another. Measure, count, weigh, and compare. Use no strength till after you have calculated the resistance it will meet. Be careful to estimate the effect before you use the means. Interest the child in never making any useless or inadequate trials of strength. If you accustom him to forecast the effect of every movement, and to correct his errors by experience, is it not certain that the more he does the better his judgment will be?
If the lever he uses in moving a heavy weight be too long, he will expend too much motion; if too short, he will not have power enough. Experience will teach him to choose one exactly suitable. Such practical knowledge, then, is not beyond his years. If he wishes to carry a burden exactly as heavy as his strength will bear, without the test of first lifting it, must he not estimate its weight by the eye? If he understands comparing masses of the same material but of different size, let him choose between masses of the same size but of different material. This will oblige him to compare them as to specific gravity. I have seen a well-educated young man who, until he had tried the experiment, would not believe that a pail full of large chips weighs less than it does when full of water.
We have not equal control of all our senses. One of them, the sense of touch, is in continual action so long as we are awake. Diffused over the entire surface of the body, it serves as a perpetual sentinel to warn us of what is likely to harm us. By the constant use of this sense, voluntary or otherwise, we gain our earliest experience. It therefore stands less in need of special cultivation. We observe however, that the blind have a more delicate and accurate touch than we, because, not having sight to guide them, they depend upon touch for the judgments we form with the aid of sight. Why then do we not train ourselves to walk, like them, in the dark, to recognize by the touch all bodies we can reach, to judge of objects around us, in short, to do by night and in the dark all they do in daytime without eyesight? So long as the sun shines, we have the advantage of them; but they can guide us in darkness. We are blind during half our life-time, with this difference, that the really blind can always guide themselves, whereas we dare not take a step in the dead of night. You may remind me that we have artificial light. What! must we always use machines? Who can insure their being always at hand when we need them? For my part, I prefer that Émile, instead of keeping his eyes in a chandler's shop, should have them at the ends of his fingers.