As was virtually pointed out by Malthus,[259] the presence of rent and interest charges in entrepreneur’s costs is an insuperable obstacle in the way of the theory that a commodity’s total labor cost is proportionate to its exchange value. If, however, an attempt is made to correlate _marginal_ labor cost and exchange value, the difficulties of rent and interest are eliminated. When we say that these difficulties are eliminated, we do not mean that they are arbitrarily set aside, or that we merely run away from them: but the marginal labor cost of a commodity is not affected by the payment of rent and interest. For instance, if wheat is being produced at the same time on land of the best and land of the poorest grade, a large rent will be paid out of the total wheat product on the former soil, and little or no rent may be paid out of the total product on the latter, and yet the cultivation will be carried to the point which makes the marginal product of labor and the marginal labor cost of wheat the same on both grades. The same observations may be applied to rent of capital (or interest, as we call it when it is calculated as a percentage of the value of the rent-bearing agent).[260] The great difficulty in the way of the theorem that the marginal labor costs of commodities are in proportion[261] to their exchange values, is the problem of skilled labor. The best way to show the effect of skilled labor upon comparative marginal costs is first to eliminate it temporarily from the problem, and show what the relation of marginal labor cost would be to exchange value, if there were only common labor throughout society. If all labor were of a single grade, all commodities which are products of labor would have exchange values in proportion to their respective marginal disutility costs. This would be true whether the products are consumption goods or are merely production goods which are used in making further products. Some valuable goods are not products of labor. Such are bodies of ore lying in their natural state, standing timber, _etc._ These production goods have no disutility cost, marginal or total, and consequently their exchange values have no relation to cost. Their supplies are determined independently of human agency. Ore at the surface, crushed or smelted ore, are, however, products of labor, and so long as only a part of the known existing ore of mines is removed—a part remaining untouched because of too high cost—the supply of any product resulting from the combination of labor and native ore-bodies, will depend upon marginal labor cost.[262]