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no need for the substitution of joint utilities for single ones. As amatter of fact the number of really perfectly completing articles isrelatively small.
If two articles are “perfect” substitutes for consumption and theratio of their marginal utilities is the same for all consumers, whilefrom a producer’s standpoint they are not “ perfect” substitutes, theconsumers fix the ratio of their prices (viz: that of their marg. ut.)and the producers produce quantities accordingly. But the quan-tities of each consumed by different individuals is entirely indeter-minate. Thus the milk from each cow may be regarded as a sep-arate commodity. The consumer is indifferent to which milk hedrinks, and purely accidental causes determine how much of each hegets ; the producer, however, milks determinate amounts from eachcow.
If two articles are perfect substitutes both for production andconsumption and the ratio of their marginal utilities and of theirmarginal disutilities are all alike their prioes will have this ratio, butthe relative quantities of each produced and consumed is entirelyindeterminate ; (e. g. the colors in the binding of a hook).
If two articles are perfect substitutes and the ratio of their mar-ginal utility of the first to the second is for every consumer greaterthan the ratio of their marginal disutilities to all producers, thefirst commodity alone will be produced and consumed and its pricewill be determined as for any commodity.
In general if two articles are perfect substitutes, but the ratio oftheir marginal utilities and the ratio of their marginal disutilities isdifferent for different individuals, those to whom the ratio of mar-ginal utilities of the first to the second is greater than the ratio oftheir prices will consume only the first, those whose utility ratio isless than the price ratio will consume only the second, those whosedisutility ratio is greater than the price ratio will produce only thesecond ; those for whom it is less, only the first.* In this case theprice of each article is determined just as usual, but for each indrvidual who does not consume or produce one or the other, its mar-ginal utility or disutility simply fails to have meaning and dropsout of the equations ; just as in Part I, occasionally a cistern maybe entirely out of the tank water.
* If some producers and consumers should have their utility or disutility ratioidentical with the price ratio the relative amounts produced and oonsume d areindeterminate to the extent of this coincidence.