The General Theory of Employment, Interest and Money
Book III: The propensity to consume

Book III of the General Theory is given over to the propensity to consume, which is introduced in Chapter 8 as the desired level of expenditure on consumption (for an individual or aggregated over an economy). The demand for consumer goods depends chiefly on the income Y and may be written functionally as C(Y). Saving is that part of income which is not consumed, so the propensity to save S(Y) is equal to Y–C(Y). Keynes discusses the possible influence of the interest rate r on the relative attractiveness of saving and consumption, but regards it as 'complex and uncertain' and leaves it out as a parameter.

His seemingly innocent definitions embody an assumption whose consequences will be considered later. Since Y is measured in wage units, the proportion of income saved is considered to be unaffected by the change in real income resulting from a change in the price level while wages stay fixed. Keynes acknowledges that this is undesirable in Point (1) of Section II.

In Chapter 9 he provides a homiletic enumeration of the motives to consume or not to do so, finding them to lie in social and psychological considerations which can be expected to be relatively stable, but which may be influenced by objective factors such as 'changes in expectations of the relation between the present and the future level of income' (p95).
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