This supposition, if ever correct, is subject to great modifications. None know better than farmers that a bushel of wheat from one field may have cost twice as much as a bushel from another field, without any possible distinction in value. Every mechanic knows that what he has accomplished with great exertion may have been duplicated by some labor-saving device with half the exertion, the two values being essentially equal. Nothing is more common than to find articles in the market sold without regard to cost because they are superseded by more desirable articles. Indeed, the most ardent defender of cost as the sole basis of value is obliged to notice multitudes of exceptions to the rule. Yet it must be granted that only those articles involving effort in securing them have value at all, and in general the amount of effort actually put forth has some relation to our estimate of value. In general, men do not exert themselves more than necessary to meet wants, and in any exchange with others estimate the value of what they have produced by the exertion expended. Yet, as products of the same kind exchange in the same market without regard to their individual cost, it is evident that some other principle must be discovered. Nevertheless, no farmer will continue indefinitely the raising of a crop which brings in the market less than a fair average return for his labor in raising it. In a series of years he expects his wheat to return a fair compensation for labor expended. In the same way every manufacturer expects a full return for all cost of all his efforts, and would not continue his work from year to year without such expectation. Moreover, when for any reason the market value of anything is much above its cost, somebody is ready to increase the supply of that particular article, and more will add their efforts in the same direction until its value approaches nearly the general cost of production as compared with the cost of other products selling in the same market.