[Footnote 145-5: The Code de Commerce, I, art. 8, requires that every merchant should keep a journal, paged and approved by the authorities, showing the receipts and disbursements of each day, on whatever account, and also the monthly expenditures of his family. Besides, he is required to make a yearly inventory of his debits and credits, subscribe to it and preserve it. That such books were excellent judicial evidence may be shown by Italian statutes of the fourteenth century. (_Martens_, Ursprung des Wechschrechts, 23.) Those of Germany even in 1449. (_Hirsch_, Danziger Handelsgeschichte, 232.)] [Footnote 145-6: Importance of the so-called "transferring to credit," where a business man considers his business as an independent entity and as distinct from himself.] SECTION CXLVI. NATIONAL INCOME.--ITS STATISTICAL IMPORTANCE. Among the most important[146-1] but also the most difficult objects of statistics, that book-keeping of nations, is national income. In estimating it, we may take our starting point from the goods which are elements of income, or from the persons who receive them as income.[146-2] In the former case the gross national income consists: A. Of the raw material newly obtained in the country. B. Of imports from foreign countries, including that which is secured by piracy, as war-booty, contributions, etc. C. The increase of values which industry[146-3] and commerce add to the first two classes up to the time of their final consumption. D. Services in the narrower sense and the produce (_Nutzungen_) of capital in use. All these several elements, estimated at their average price in money, which supposes that all purchases, especially those under the head D, are made voluntarily[146-4] and at their natural price. To find the national net income, we must deduct the following items: A. All the material employed in production which yields no immediate satisfaction to any personal want.[146-5] B. The exports which pay for the imports. C. The wear and tear of productive capital and capital in use. In the second case the net national income is to be calculated from the following items: A. From the net income of all independent private businesses etc.[146-6] B. From the net income of the state, of municipalities, corporations and institutions, derived from their own resources. C. Under the former heads must be taken into the account such parts of property as have been immediately consumed and enjoyed.[146-7]