If the rate of surplus-value is known and its magnitude given, the rate of profit expresses nothing but what it actually is, namely a different way of measuring surplus-value, its measurement according to the value of the total capital instead of the value of the portion of capital from which surplus-value directly originates by way of its exchange for labour. But in reality (i.e., in the world of phenomena) the matter is reversed. Surplus-value is given, but given as an excess of the selling price of the commodity over its cost-price; and it remains a mystery where this surplus originated — from the exploitation of labour in the process of production, or from outwitting the purchaser in the process of circulation, or from both. What is also given is the proportion of this surplus to the value of the total capital, or the rate of profit. The calculation of this excess of the selling price over the cost-price in relation to the value of the advanced total capital is very important and natural, because in effect it yields the ratio in which total capital has been expanded, i.e., the degree of its self-expansion. If we proceed from this rate of profit, we cannot therefore conclude the specific relations between the surplus and the portion of capital invested in wages. We shall see in a subsequent chapter [K. Marx, Theorien über den Mehrwert. K. Marx/F. Engels, Werke, Band 26, Teil 3, S. 25-28 . — Ed.] what amusing somersaults Malthus makes when he tries in this way to get at the secret of the surplus-value and of its specific relation to the variable part of the capital. What the rate of profit actually shows is rather a uniform relation of the surplus to equal portions of the total capital, which, from this point of view, does not show any inner difference at all, unless it be between the fixed and circulating capital. And it shows this difference, too, only because the surplus is calculated in two ways; namely, first, as a simple magnitude — as excess over the cost-price. In this, its initial, form, the entire circulating capital goes into the cost-price, while of the fixed capital only the wear and tear goes into it. Second, the relation of this excess in value to the total value of the advanced capital. In this case, the value of the total fixed capital enters into the calculation, quite the same as the circulating capital. Therefore, the circulating capital goes in both times in the same way, while the fixed capital goes in differently the first time, and in the same way as circulating capital the second time. Under the circumstances the difference between fixed and circulating capital is the only one which obtrudes itself.