Keynes postulated that aggregate consumption is a function of aggregate current disposable income. The Keynesian consumption function is written as: C = a + cY a > 0, 0 < c < 1; where a is the intercept, a constant which measures consumption at a zero level of disposal income; c is the marginal propensity to consume (MPC); and Y is the disposal income. So as income increases, average propensity to consume (APC = C/ Y) falls.