In Section Two he discusses the flip side of this duality. First, every thing requires labor to be useful. The apple must be picked, the corn must be planted and reaped, the cotton must be picked and weaved, the coat must be sewn. Labor is what makes the thing useful. Second, labor has value, and it is labor that gives value to things.
The amount of unskilled labor used to create a thing is the basic measure of value. Skilled labor can be measured in unskilled labor units. Thus an hour of skilled labor is worth more than an hour of unskilled labor in accordance with social conventions.
A thing's useful value (as opposed to money value) has two components: (1) the raw materials that go into the thing; and (2) the labor that goes into shaping the raw materials into a useful thing or commodity. However, a thing has monetary value only to the extent that it was labored upon.
"Tailoring and weaving, though qualitativley different productive acitivies, are each a productive expenditure of human brains, nerves and muscles, and in this sense are human labor . They are but two different modes of expending human labor-power. . . . But the evalue of a commodity represents human labor in the abstract, the expenditure of human laobr in general. And just as in society, a general or a banker plays a great part, but mere man, on the other hand, a very shabby part, so here with mere humnan labor... Skilled labor counts only as a simple labor intensified, or rather, as multiplied simple labor, a given quanitty of skille being consdered equal to a greater quantity of simple labor.
Nature, it seems, is left out of the equation. Although raw materials are "useful", only labor adds value. Marx would say that when we buy lumber to build a house the price of the lumber is a function of the labor that went into cutting down the tree and shaping it into a 2x4. We are just now, I think, beginning to appreciate the value of natural resources (like clean air, stable envirionment, etc.).
The idea of a fungible unit of unskilled or basic labor, with an assumed baseline rate of productivety, seems like some type of Platonic form. Marx uses an example of a coat incorporating twice as much labor as the cloth of which it's made. The measure of this exact relationship would seem to be highly impractical,nay impossible to measure in practice. The same goes for determining the multiple to be applied to different forms of skilled labor. Marx mentions "social conventions." That sounds very much like the whole thing is still fundamentally market driven.