As exchange-values, all commodities are merely definite quantities of congealed labor-time.

— Karl Marx

A use-value, or useful article, therefore, has value only because abstract human labor is objectified or materialized in it. How, then, is the magnitude of this value to be measured? By means of the quantity of the "value-forming substance", the labor, contained in the article. This quantity is measured by its duration, and the labor-time is itself measured on the particular scale of hours, days etc.

— Karl Marx

A use-value, or useful article, therefore, has value only because abstract human labor is objectified or materialized in it. How, then, is the magnitude of value to be measured? By means of the quantity of the "value-forming substance", the labor, contained in the article. This quantity is measured by its duration, and the labor-time is itself measured on the particular scale of hours, days etc.

— Karl Marx

What exclusively determines the magnitude of the value of any article is therefore the amount of labor socially necessary, or the labor-time socially necessary for its production.

— Karl Marx