(7) Such an equilibrium picture leaves untouched the vital question which any theory must answer which means to be of practical use in concrete situations: what are the real _variables_ in the situation, and what factors are constant? What causes are _likely_ to produce changes in market prices? The individual-utility curves, which in Austrian theory are commonly treated as the only variables, except quantities of goods,--in the strict static picture there are no variables at all!--are really, when conceived of as individual, as growing out of the mental processes of each individual separately, the most _constant_ factor in the situation. For, on the principle of the inertia of large numbers, each unit of which is moved by its own peculiar causes, changes in the utility-curves of one man will be offset by opposite changes in the utility-curves of another, and so the general system will remain much where it was. Of course, if a rich man changes his curve, a poor man's change will not offset it in the market, but this is to emphasize the _distribution of wealth_ rather than the utility-curves. It is only when you get changes of a sort that the individualistic psychology, and the "pure economic" explanation factors, of the Austrians find no place for, that you can predict a change in the general price-system. It is only changes in fashion or mode, in general business confidence,[86] in moral attitude toward this or the other sort of consumption or production, in the distribution of wealth, changes in taxes and other laws--causes of a general social character--that you can count on to produce important changes in values. Of course, changes in the adequacies of supplies would be taken account of on either interpretation.