D. Interest on debt must be added only on the side of the creditor, and deducted from the income of the debtor; otherwise, _error dupli_. This does not apply to taxes or church dues because the subjects of a good state and members of a good church purchase thereby things which are really new and of at least equal value to the outlay. Besides, in both instances, it is necessary to calculate the number of men who live from the national income, the average amount of their indispensable wants, and the average price in money of the same, in order to determine the _free_ national income by deducting the sum total of these average wants, estimated at this average price.[146-8] [146-9] [Footnote 146-1: Not only to compare the happiness and power of different nations with one another, but also for purposes of taxation, the profitableness and innocuousness of which suppose the most perfect adaptation to the income of the whole people.] [Footnote 146-2: The former, in _Rau_, Lehrbuch, I, § 247; the latter in _Hermann_, 308 ff. The former mode of calculation gives us a means of judging of the comfort of the people, their control of natural forces, etc.; the second, of the relation of classes among the people. (_v. Mangoldt_, Grundriss, 99. V. W. L., 316 ff.) Each member of the nation produces his income only in the whole of the nation's economy. Hence _Held_, Die Einkommensteuer, 1872, 70, 77, would, but indeed only under very abstract fictions, construct private income from the national, and not _vice versa_.] [Footnote 146-3: On the average degree of this increase of values in different industries, see _Chaptal_, De l'Industrie française, II, passim. _Bolz_, Gewerbekalender für, 1833, 111. No such scale can be lastingly valid, because, for instance, almost all technic progress decreases the appreciation of values through industry, and every advance made by luxury raises the claims to refined quality etc. See _Hildebrand_, Jahrbücher für Nat-Oek., 1863, 248 ff.] [Footnote 146-4: Many items in Class D evade all calculation. Thus, for instance, the numberless cases of personal services which are enjoyed only by the doer himself; also the greater number of products (_Nutzungen_=usufruct) of capital in use for the consumption of the owner himself. (Latent income.) Only, it may be, in the case of dwelling houses, equipages, etc., that the consumption by use can be estimated in accordance with the analogy of similarly rented goods.] [Footnote 146-5: The principal materials consumed in manufactures are of course not to be deducted here, because the increase in their value was taken into account above.]