Welfare, however, is a thing of very wide range. There is no
need here to enter upon a general discussion of its content. It
will be sufficient to lay down more or less dogmatically two
propositions; first, that the elements of welfare are states of
consciousness and, perhaps, their relations; secondly, that
welfare can be brought under the category of greater and less.
A general investigation of all the groups of causes by which
welfare thus conceived may be affected would constitute a task
so enormous and complicated as to be quite impracticable. It is,
therefore, necessary to limit our subject-matter. In doing this
we are naturally attracted towards that portion of the field in
which the methods of science seem likely to work at best
advantage. This they can clearly do when there is present
something measurable, on which analytical machinery can get a
firm grip. The one obvious instrument of measurement available
in social life is money. Hence, the range of our inquiry becomes
restricted to that part of social welfare that can be brought
directly or indirectly into relation with the measuring-rod of
money. This part of welfare may be called economic
welfare. …Nevertheless, though no precise boundary
between economic and non-economic welfare exists, yet the test
of accessibility to a money measure serves well enough to set up
a rough distinction. Economic welfare, as loosely defined by
this test, is the subject-matter of economic science. The
purpose of this volume is to study certain important groups of
causes that affect economic welfare in actual modern
societies
Pigou (1932 [1920]) p. 13–14