It would, perhaps, not be very difficult to argue that a universal theory of price is impossible. The attempt to base an _entire_ economy on the motive of Self-Interest has not been so successful, that many of us are willing to risk the credit of the whole science any longer on an assumption that was never quite true, and is becoming less so as wealth increases and is increasingly spent with a directly moral aim. But, in certain great departments of exchange, if anywhere, the old competitive laws do hold. In stock exchange dealings, in banking, in international transactions, in great organised markets, as iron, wool, cotton, grain, and so on, the egoistic motive is so strongly marked that it is possible to found on it a law which comes, perhaps, as near a scientific law of exchange as we can expect. It may be described as the law of price under perfect competition. It disregards all motives but those of _advantage from the exchange_—always, of course, within the recognised limits of law and respectability. In such markets the “strong” exchanger (buyer or seller) is the one who attaches most importance to the good he wishes to get, and the least importance to the good he gives in exchange—as we can see from the simple consideration that the bidder most likely to carry away a picture from a studio is the one who thinks most of the picture and least of his money, while the artist most likely to clear his stock is the one who thinks least of his pictures and most of the money he will get for them. The assumptions on which the law is based are the following: that the market is an open and organic one; that buyers and sellers are ordinarily conversant with the conditions of supply and competition; that each party will make an exchange whenever he sees a gain in it; and that he will prefer a greater gain to a less. They are the assumptions of any ordinary commercial “market.”[15] For simplicity’s sake, we shall begin with the simplest possible case, and gradually come to the more complicated.