In the diagram above, something of what is to follow is anticipated, since the cause of value is indicated. Wheat is shown to be exerting an influence on milk, and milk exerts an influence on wheat. The comparative strength of these two influences determines the ratio of exchange between them. But these two influences are not ultimate. The ratio of exchange is a relation, a _reciprocal_ relation. It works both ways. But behind this relativity, this scheme of relations between values, there lie two values which are absolute. These values rest in the pull exerted on wheat and on milk by the human factor which is fundamental, which in our diagram we have called the "social mind." Values lie behind ratios of exchange, and causally determine them. The important thing for present purposes is merely to note that value is prior to exchange relations, that it is an absolute quantity, and not, as many economists have put it, purely relative. The ratio of exchange is relative, but there must be absolutes behind relations. A _price_ is merely one particular kind of ratio of exchange, namely, a ratio of exchange in which one of the terms is the value of the money unit.[6] In modern life, prices are the chief form of ratio of exchange, but it is important for some purposes to remember that they are not the only form. Values may simultaneously rise and fall. There may be an increase or decrease in the sum total of values. Ratios of exchange cannot all rise or fall. A rise in the ratio of the value of wheat to the value of milk means a fall in the ratio of the value of milk to the value of wheat. Both may have fallen in absolute value, but both cannot simultaneously rise or fall with reference to one another. This is the truism regarding ratios of exchange which many economists have inaccurately applied to value itself in the doctrine that there cannot be a simultaneous rise or fall of values. There can be a simultaneous rise or fall of values, but not a simultaneous rise or fall of ratios of exchange.