During the second period of the labor-process, that in which his labor is no longer necessary labor, the workman, it is true, labors, expends labor-power; but his labor, being no longer necessary labor, he creates no value for himself. He creates surplus-value which, for the capitalist, has all the charms of a creation out of nothing. This portion of the working day, I name surplus labor-time, and to the labor expended during that time, I give the name of surplus labor. It is every bit as important, for a correct understanding of surplus-value, to conceive it as a mere congelation of surplus labor-time, as nothing but materialized surplus labor, as it is, for a proper comprehension of value, to conceive it as a mere congelation of so many hours of labor, as nothing but materialized labor. The essential difference between the various economic forms of society, between, for instance, a society based on slave-labor, and one based on wage labor, lies only in the mode in which this surplus labor is in each case extracted from the actual producer, the laborer. Marx, Capital Volume 1 (Progress Publishers, 1887), p. 152-153