It is not easy, it has already been observed, to ascertain what are the average wages of labour, even in a particular place, and at a particular time. We can, even in this case, seldom determine more than what are the most usual wages. But even this can seldom be done with regard to the profits of stock. Profit is so very fluctuating, that the person who carries on a particular trade, cannot always tell you himself what is the average of his annual profit. It is affected, not only by every variation of price in the commodities which he deals in, but by the good or bad fortune both of his rivals and of his customers, and by a thousand other accidents, to which goods, when carried either by sea or by land, or even when stored in a warehouse, are liable. It varies, therefore, not only from year to year, but from day to day, and almost from hour to hour. To ascertain what is the average profit of all the different trades carried on in a great kingdom, must be much more difficult; and to judge of what it may have been formerly, or in remote periods of time, with any degree of precision, must be altogether impossible. But though it may be impossible to determine, with any degree of precision, what are or were the average profits of stock, either in the present or in ancient times, some notion may be formed of them from the interest of money. It may be laid down as a maxim, that wherever a great deal can be made by the use of money, a great deal will commonly be given for the use of it; and that, wherever little can be made by it, less will commonly be given for it. Accordingly, therefore, as the usual market rate of interest varies in any country, we may be assured that the ordinary profits of stock must vary with it, must sink as it sinks, and rise as it rises. The progress of interest, therefore, may lead us to form some notion of the progress of profit.