In the strictest sense, accordingly, there is no such thing as physical labor, because the mental must coöperate with the physical even in the lowest forms of human exertion; and in the same sense there can be no such thing as exclusively mental labor, for the bodily powers conspire more or less in the highest intellectual efforts that are ever sold. Nevertheless, both the phrases, physical labor and mental labor, are convenient and not harmful, whenever on the one side the bodily powers seem to be predominant in the effort, and on the other the intellectual. It is now to be noticed, that all that men can do, when they labor physically, is _to move something_. When a man works with his hands or his feet or his whole body, all that he does or can do, is to begin a series of motions or resistances to motion, for this good reason, human muscles in their very structure are capable only of starting motion and stopping motion. All the marvellous results of physical effort in all the world have flowed from so simple a matter as the contraction and expansion of muscle; and the world of materials is so cunningly constructed, that, when these are moved into right position by human hands, or by some form of capital itself the result of previous human handling, the free powers of Nature do all the rest, and valuable commodities are the good outcome. For one example, when the woodman fells a tree for sale, he brings a series of motions (_labor_) to bear upon the trunk, by means of his sharp axe (_capital_), and then the power of gravitation (_nature_) seizes the tree and brings it crashing to the earth. For a second illustration, wool and cotton have by nature a certain tenacity of fibre, and what is more to the point, a certain _kinkiness_ of fibre easily interlinking one with another indefinitely in length; men move these separate fibres in certain relations to each other by an instrument (_capital_) called a spindle, and the result is thread; then other men move these threads into relations with each other by means of an implement (_capital_) called a shuttle, and the outcome is a web of cloth; lastly, the tailor moves his shears through the cloth, and then his needles, and the issue is a coat, a commodity, the valuable for which all these processes were gone through with, and by the sale of which all the onerous factors therein are compensated.