Keynes showed that savings (S) are determined by income (Y) rather than interest rates. When income is zero, consumption cannot be zero so consumers must borrow or use past savings, resulting in negative or "dissavings" (S<0). As income increases, both consumption (C) and savings increase, but savings increase at a lower rate. The average propensity to save (APS) represents savings (S) as a proportion of income (Y) and increases as income rises from dissaving to positive saving. The marginal propensity to save (MPS) represents the increase in savings from a small increase in income and remains constant as the savings function is a straight line.