CB is consequently the time in which the surplus-value is produced. The aim of the capitalist, then, is to protract the period CB as much as possible, and this he can accomplish in two ways: first, that of which we have just spoken, the prolongation of AB; second, the shortening of AC in order that CB may be as long as possible, or in other words, the shortening of the time necessary for the production of the necessaries of life for the workman and his family. When the productivity of labor rises, the value of the commodities falls; and when the value of the commodities which the workman needs for his support falls, the value of labor falls also. However, the increase of the productivity of labor is only possible through the improvement of the means of production and the methods of working, so that capitalism modifies its manner of production unceasingly. In general the capitalist does not take account of the fact that the more cheaply commodities are produced for sale, the more the value of labor falls, and, other things being equal, the more the surplus-value increases. The capitalist strives constantly to make improvements in his methods of production in order to surpass his competitors. Supposing that, by the employment of a new method of working, he succeeds in producing in half an hour an article up to that time generally made in an hour, he will then obtain an extra profit as long as his competitors do not employ this method. But as soon as the latter have improved their production in the same way, the time socially necessary falls from an hour to half an hour, and the extra profit of the man who first introduced the method ceases. The result that remains is this: the value of labor has decreased, and the relative surplus-value has consequently increased, in so far as the commodities whose value has declined are destined to provide for the needs of the workmen. Before entering upon the consideration of some of the methods for shortening the necessary hours of labor, attention must be fixed upon the fact that, aside from this method, the capitalist tries above all to lower the price of labor below its value, while the workers, for their part, oppose this tendency and try to obtain the contrary; whence there results a new cause for an inexorable struggle between labor and capital, side by side with that over the length of the working-day.