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|on the use of the differential calculus etc.||81|
This description is applicable generally to the subject designated by the main portion of my title — «the use of the Differential Calculus» — and more particularly to that part of the subject which is demarcated by the limiting clause — «to determine conditions of maximum advantage». The subject thus limited is, if we understand «economics» in its largest sense, almost coincident with — only slightly narrower than — the field which is covered by the well-known dictum of Malthus: «many of the questions, both in morals and politics, seem to be of the nature of the problems de maximis et minimis in fluxions». Such is the little territory, on the borderland between Physics and Psychology, which I attempt here to survey.
One main division has already come into view, that which separates Economics in a narrow or «proper» sense from the wider Science of Political or rather Social Economy, of which the object is not simply wealth, but welfare, so far as dependent on wealth. We might define Economics proper — the Subject of our Section I — by the limitation that in Jevons’s words «the motive in one mind is weighed only against other motives in the same mind, never against the motives in other minds»1. Whereas in Social Economy — the Subject of Section II — what Jevons calls «a common denomination of feeling» must be postulated.
According to the postulate, the relation between the individuals of a Society may be described by the familiar metaphor of solidarity. But the rmysical analogy of economics proper does not lie so ready to hand. The particles of an economic system neither cohere as a solid, nor collide with the independence of a gas. Their liquid movements are comparable to a dance in which youths and maidens move in unison; harmoniously, but subject to a change of partners.
To learn the steps of this peculiar dance consider first the movement of a single couple. One of the pair, say X, gives to the other, say Y, a quantity x of the commodity x in return for the amount y of the commodity y given by Y to X. The terms of such an arrangement may be represented by a point (x, y) on the plane of the paper. The variation
- Theory of Political Economy, edition 3, p. 14. Compare Pareto, .