Pigou et al. are more or less utilitarian. They spoke in terms of total social utility without a way to account for tradeoffs between individuals. This assumption was strongly attacked by e.g. Robbins (1932) and others. The New Welfare Economics arose in the 1930s, replacing total welfare with Pareto optimality. This was made possible by the developments in consumer theory we have previously surveyed.
Introspection does not enable A to know what it going on in B's
mind, nor B to measure what is going on in A's. There is no way
of comparing satisfactions of different people.
And supposing [we had shown] that certain policies had the
effect of increasing 'social utility', even so it would
be totaly illigitimate to argue that such an conclusion …
warranted the inference that these policies ought to be
carried out.
Robbins, Essay p. 141-2
