(The argument of Duehring against which Engels here directs his efforts may be best summed up in Duehring's concluding words "Marx in his utterances on value cannot escape the lurking ghost of highly skilled labor. The prevalent notion of the intellectual classes has been a hindrance to him in this matter, for according to this idea it is an enormity to reckon the labor time of a barrow pusher and an architect as economic equivalents.") Engels thereupon says "the passage in the works of Marx which caused this outbreak on the part of Duehring is very short." Marx is examining the question as to the basis of the value of commodities and answers it by the statement that it is the amount of human labor contained in them. "This" he goes on "is the expression of that simple labor force which belongs to the average human being without any special development. Skilled labor is a power or rather a multiple of simple labor, so that a small amount of skilled labor is equivalent to a larger amount of unskilled labor. Practice shows that this reduction to the terms of unskilled labor takes place. A commodity may be the product of skilled labor, its value may be equivalent to a product of unskilled labor skilled labor. The proportion in which different forms of labor are reduced to their general standard in unskilled labor is established by a social process going on behind the backs of the producers, and appears to them merely customary." Here Marx is only dealing with the value of commodities, that is of objects produced and exchanged by private producers in a society consisting of private producers producing for their own profit. He is therefore not concerned here with "absolute value" whatever that may be but only with the value which is realised in a given form of society. This value under the given social conditions is shaped and measured by the human labor incorporated in the commodities and this human labor shows itself as the expression of simple human energy. But every piece of work is not merely an expression of simple labor force. Very many labor products require the expenditure of more or less time, money, trouble, and acquired skill or knowledge. Do these kinds of compound labor show at the same period of time the same commodity values as simple labor, are they the expression of merely simple labor force? Evidently not. The product of an hour of compound labor is a commodity of higher, double or three times the value of a product of an hour of simple labor. The value of the product of compound labor can in this comparison be expressed through the measure of simple labor; and this reduction of compound labor is carried on by means of a social progress behind the back of the producer, by means of which can here be established according to the theory of value but not explained. The thing which Marx states here is a simple fact which happens every day before our eyes in the present capitalistic society.