Surely then some distinction between the different kinds of labour, with reference to their different effects on national wealth, must be admitted to be not only useful, but necessary; and if so, the question is what this distinction should be, and where the line between the different kinds of labour should be drawn.
Book I, Chapter I, Of The Definitions of Wealth and of Productive Labour, Section II, p. 43. - Principles of Political Economy (Second Edition 1836)
