It is, however, difficult to avoid another error. When the master is very fond of certain occupations, he is apt to assume that the child shares his tastes; beware lest you are carried away by the interest of your work, while the child is bored by it, but is afraid to show it. The child must come first, and you must devote yourself entirely to him. Watch him, study him constantly, without his knowing it; consider his feelings beforehand, and provide against those which are undesirable, keep him occupied in such a way that he not only feels the usefulness of the thing, but takes a pleasure in understanding the purpose which his work will serve. The solidarity of the arts consists in the exchange of industry, that of commerce in the exchange of commodities, that of banks in the exchange of money or securities. All these ideas hang together, and their foundation has already been laid in early childhood with the help of Robert the gardener. All we have now to do is to substitute general ideas for particular, and to enlarge these ideas by means of numerous examples, so as to make the child understand the game of business itself, brought home to him by means of particular instances of natural history with regard to the special products of each country, by particular instances of the arts and sciences which concern navigation and the difficulties of transport, greater or less in proportion to the distance between places, the position of land, seas, rivers, etc. There can be no society without exchange, no exchange without a common standard of measurement, no common standard of measurement without equality. Hence the first law of every society is some conventional equality either in men or things. Conventional equality between men, a very different thing from natural equality, leads to the necessity for positive law, i.e., government and kings. A child's political knowledge should be clear and restricted; he should know nothing of government in general, beyond what concerns the rights of property, of which he has already some idea. Conventional equality between things has led to the invention of money, for money is only one term in a comparison between the values of different sorts of things; and in this sense money is the real bond of society; but anything may be money; in former days it was cattle; shells are used among many tribes at the present day; Sparta used iron; Sweden, leather; while we use gold and silver. Metals, being easier to carry, have generally been chosen as the middle term of every exchange, and these metals have been made into coin to save the trouble of continual weighing and measuring, for the stamp on the coin is merely evidence that the coin is of given weight; and the sole right of coining money is vested in the ruler because he alone has the right to demand the recognition of his authority by the whole nation.