While economic theory in general may be defined as the theory of how an economic condition or an economic development is determined within an institutional framework, the deals with how to judge whether one condition can be said to be better in some way than another and whether it is possible, by altering the institutional framework, to achieve a better condition than the present one.
There is much of economic theory which is pursued for no better reason than its intellectual attraction; it is a good game. We have no reason to be ashamed of that, since the same would hold for many branches of pure mathematics.
The purpose of income calculations in practical affairs is to give people an indication of the amount which they can consume without impoverishing themselves. Following out this idea, it would seem that we ought to define a man's income as the maximum value which he can consume during a week, and still expect to be as well off at the end of the week as he was at the beginning.
Income No. 3 must be defined as the maximum amount of money which the individual can spend this week, and still be able to spend the same amount in real terms in each ensuing week.
So long as we confine our attention to income from property, and leave out of account any increment or decrement in the value of prospects due to changes in people's own earning power (accumulation or decumulation of “Human Capital”), Income No. 1 ex post is not a subjective affair, like other kinds of income; it is almost completely objective.
The standard stream corresponding to Income No. 3 is constant in real terms... We ask... how much he would be receiving if he were getting a standard stream of the same present value as his actual expected receipts. This amount is his income.
It is not so well known that it [Keynes's and my own move from thinking in terms of price-levels and the rate of interest to thinking in terms of inputs and outputs] is matched by a movement from Hayek to Harrod. I once asked Harrod what had put him on to the construction of his so-call 'dynamic' theory; he said, to my surprise, that it was thinking about Hayek.
There were four years, 1931-1935, when I was myself a member of Hayek's seminar in London; it has left a deep mark on my thinking... At the end of the discussions in that seminar … we were, I believe, on the point of taking what now seems to me to be a decisive step. I was, at least, on the point of taking it myself. There is evidence for that in my "Value and Capital", much of the groundwork for which was done before I left London.
Hayek was making us think of the productive process as a process in time, inputs coming before outputs.
I did not begin from Keynes: I began from Pareto, and Hayek.