104 , Irving Fisher—Mathematical investigations
again a physical analogue. Water seeks its level, but this law doesnot fully explain Niagara. A great deal of special data are herenecessary and the physicist is as unfit to advise the captain of theMaid of the Mist as an economist to direct a Wall street speculator.The failure to separate statics from dynamics appears historically*to explain the great confusion in early physical ideas. To makethis separation required the reluctant transition from the actualworld to the ideal. The actual world both physical and economichas no equilibrium. “ Normal”! price, production and consumptionare sufficiently intricate without the complication of changes in socialstructure. Some economists object to the notion of “ normal ” as anideal but unattainable state They might with equal reason objectto the ideal and unattainable equilibrium of the sea.
The dynamical side of economics has never yet received system-atic treatment. When it has, it will reconcile much of the presentapparent contradiction, e. g. if a market is out of equilibrium, thingsmay sell for “ more than they are worth,” as every practical manknows, that is the proper ratios of marginal utilities and prices arenot preserved.
§0.
We have assumed a constant population. But population doeschange and with it all utility functions change. An analysis whoseindependent variable is population^ leads to another department ofeconomics. In the foregoing investigation the influence of popula-tion was included in the form of the utility function. So also withall causes physical, mental and social not dependent on the quantitiesof commodities or services.
§ 7 .
Individuals are not free to stop consuming or producing at anypoint. Factory operatives must have uniform working hours. Themarginal undesirability of the last hour may for some workmenequal, for others exceed or fall short of the utility of its wages.
§ 8 .
No one is fully acquainted.with all prices nor can he adjust hisactions to them with the nicety supposed ; both these considerationsare starting points for separate discussion.