"We are sensible of the position of our limbs; we know that the arms hang by the sides, or that they are raised and held out, although we touch nothing and see nothing. It must be a property internal to the frame by which we thus know the position of the members of our body; and what can it be but a consciousness of the degree of action, and of the adjustment of the muscles? At one time, I entertained a doubt whether this proceeded from a knowledge of the condition of the muscles, or from a consciousness of the degree of effort which was directed to them in volition. It was with a view to elucidate this that I made the observations which terminated in the discovery that every muscle had two nerves; one for sensation, and one to convey the mandate of the will, and direct its action. I had reasoned in this manner: we awake with a knowledge of the position of our limbs; this cannot be from a recollection of the action which placed them where they are; it must, therefore, be a consciousness of their present condition. When a person in these circumstances moves, he has a determined object, and he must be conscious of a previous condition before he can desire a change, or direct a movement." In walking, riding, swimming, and other bodily exercises; in writing, playing on the piano, or any other instrument of music, the muscular sense is called into activity, and co-operates with the senses of sight, touch, etc. Muscular exertion is in itself pleasurable, when not pushed too far; and, indeed, the healthy condition, both of the body and mind, results from muscular exertion, and the alternations of activity and repose. The appointment of man, after the fall, to a life of labour, was a judgment tempered by mercy. Man is not destined to lead a life of sloth or supineness, but, urged on by his natural wants, he cultivates the ground, he tames the wild beast of the forest or mountain, he builds houses, he constructs ships, he clothes himself with fabrics, he surrounds himself with the comforts, the decencies, the luxuries of life, and in every work which his hands find to do his muscular sense is in requisition; it is this sense which appreciates resistance, which enables us to balance our bodies, to move with gracefulness, to run, to walk, to leap, to throw the shuttle with precision, or wield the hammer with skill. It is improved by education; the child learns first to walk, and then to run and leap, and in due time to know its own strength, and also the best mode of employing that strength, without reflecting upon the laws of mechanics, or any philosophic theories. It is evident, then, that we have "a perception of the condition of the muscles previous to the exercise of the will," and that with respect to the hand, "it is not more the freedom of its action which constitutes its perfection, than the knowledge which we have of these motions, and our consequent ability to direct it with the utmost precision."