I contend that Keynes provides two contradictory definitions of aggregate income. According to the first definition, which is the dominant in Keynes as well as the standard in current Macroeconomics, the full value of output becomes income in the aggregate. This view can be traced back, at least, to Adam Smith. According to the second definition, on the contrary, not the full value of output becomes income, but only the part of it not required to make up for capital consumption. This view can be traced back to the Physiocrats. In the ¿General Theory¿, Keynes inconsistently appeals to these two contrary views, as I show by analyzing his treatment of the concept of ¿user cost¿. In chapter 3, user cost becomes income and investment gives rise ...