“The wealth of those societies in which the capitalist mode of production prevails, presents itself as ‘an immense accumulation of commodities…’ A commodity is, in the first place, an object outside us, a thing that by its properties satisfies human wants of some sort or another. …

The utility of a thing makes it a use value. But this utility is not a thing of air. Being limited by the physical properties of the commodity, it has no existence apart from that commodity. A commodity, such as iron, corn, or a diamond, is therefore, so far as it is a material thing, a use value, something useful. This property of a commodity is independent of the amount of labour required to appropriate its useful qualities. … Use values become a reality only by use or consumption.

Exchange value… [is] the proportion in which values in use of one sort are exchanged for those of another sort, a relation constantly changing with time and place. … If we then leave out of consideration the use value of commodities, they have only one common property left, that of being products of labour. But even the product of labour itself has undergone a change in our hands. … Along with the useful qualities of the products themselves, we put out of sight both the useful character of the various kinds of labour embodied in them, and the concrete forms of that labour; there is nothing left but what is common to them all; all are reduced to one and the same sort of labour, human labour in the abstract.”

Karl Marx, Capital (Das Kapital)