However, the mere existence of an excess in the value of agricultural products over their price of production would not in itself suffice to explain the existence of a ground-rent which is independent of differences in fertility of various soil types and in successive investments of capital on the same land — a rent, in short, which is to be clearly distinguished in concept from differential rent and which we may therefore call absolute rent. Quite a number of manufactured products are characterised by the fact that their value is higher than their price of production, without thereby yielding any excess above the average profit, or a surplus-profit, which could be converted into rent. Conversely, the existence and concept of price of production and general rate of profit, which it implies, rest upon the fact that individual commodities are not sold at their value. Prices of production arise from an equalisation of the values of commodities. After replacing the respective capital-values used up in the various spheres of production, this distributes the entire surplus-value, not in proportion to the amount produced in the individual spheres of production and thus incorporated in their commodities, but in proportion to the magnitude of advanced capitals. Only in this manner do average profit and price of production arise, whose characteristic element the former is. It is the perpetual tendency of capitals to bring about through competition this equalisation in the distribution of surplus-value produced by the total capital, and to overcome all obstacles to this equalisation. Hence it is their tendency to tolerate only such surplus-profits as arise, under all circumstances, not from the difference between the values and prices of production of commodities but rather from the difference between the general price of production governing the market and the individual prices of production differing from it; surplus-profits which obtain within a certain sphere of production, therefore, and not between two different spheres, and thus do not affect the general prices of production of the various spheres, i.e., the general rate of profit, but rather presuppose the transformation of values into prices of production and a general rate of profit. This supposition rests, however, as previously discussed, upon the constantly changing proportional distribution of the total social capital among the various spheres of production, upon the perpetual inflow and outflow of capitals, upon their transferability from one sphere to another, in short, upon their free movement between the various spheres of production, which represent so many available fields of investment for the independent components of the total social capital. The premise in this case is that no barrier, or just an accidental and temporary barrier, interferes with the competition of capitals — for instance, in a sphere of production, in which the commodity-values are higher than the prices of production, or where the surplus-value produced exceeds the average profit — to reduce the value to the price of production and thereby proportionally distribute the excess surplus-value of this sphere of production among all spheres exploited by capital. But if the reverse occurs, if capital meets an alien force which it can but partially, or not at all, overcome, and which limits its investment in certain spheres, admitting it only under conditions which wholly or partly exclude that general equalisation of surplus-value to an average profit, then it is evident that the excess of the value of commodities in such spheres of production over their price of production would give rise to a surplus-profit, which could be converted into rent and such made independent with respect to profit. Such an alien force and barrier are presented by landed property, when confronting capital in its endeavour to invest in land; such a force is the landlord vis-à-vis the capitalist.