Attention has already (pp.100, 105) been called to the fact that for some purposes such things as tea and coffee must be grouped together as one commodity: and it is obvious that, if tea were inaccessible, people would increase their consumption of coffee, and vice versa.The loss that people would suffer from being deprived both of tea and coffee would be greater than the sum of their losses from being deprived of either alone: and therefore the total utility of tea and coffee is greater than The sum of the total utility of tea calculated on the supposition that people can have recourse to coffee, and that of coffee calculated on a like supposition as to tea.This difficulty can be theoretically evaded by grouping the two "rival" commodities together under a common demand schedule.On the other hand, if we have calculated the total utility of fuel with reference to the fact that without it we could not obtain hot water to obtain the beverage tea from tea leaves, we should count something twice over if we added to that utility the total utility of tea leaves, reckoned on a similar plan.Again the total utility of agricultural produce includes that of ploughs; and the two may not be added together; though the total utility of ploughs may be discussed in connection with one problem, and that of wheat in connection with another.Other aspects of these two difficulties are examined in V, VI.
Prof.Patten has insisted on the latter of them in some able and suggestive writings.But his attempt to express the aggregate utility of all forms of wealth seems to overlook many difficulties.
8.In mathematical language the neglected elements would generally belong to the second order of small quantities; and the legitimacy of the familiar scientific method by which they are neglected would have seemed beyond question, had not Prof.
Nicholson challenged it.A short reply to him has been given by Prof.Edgeworth in the Economic Journal for March 1894; and a fuller reply by Prof.Barone in the Giornale degli Economisti for Sept.1894; of which some account is given by Mr Sanger in the Economic Journal for March 1995.
As is indicated in Note VI in the Mathematical Appendix, formal account could be taken of changes in the marginal utility of money, if it were desired to do so.If we attempted to add together the total utilities of all commodities, we should be bound to do so: that task is however impracticable.
9.The notion of consumers' surplus may help us a little now;and, when our statistical knowledge is further advanced, it may help us a great deal to decide how much injury would be done to the public by an additional tax of 6d.a pound on tea, or by an addition of ten per cent.to the freight charges of a railway:
and the value of the notion is but little diminished by the fact that it would not help us much to estimate the loss that would be caused by a tax of 30s.a pound on tea, or a tenfold rise in freight charges.
