At a dinner with Sims, when I was just coming out of graduate school, I made some mention of aggregate demand. He asserted that there was no such thing.
The always interesting Eric Falkenstein says:
One characteristic of Keynesian thinking is to think the problem currently is with inadequate Aggregate Demand, as if this is some simple analytical construct that is just as meaningful as the demand for apples. This is nonsense. Aggregate Demand and Aggregate Supply are incoherent constructs that require heroic assumptions. You might as well talk about the current law of motion on the Hegelian Dialectic, which for decades was discussed as if it were real. Things exist before people know they exist (e.g., nations, atoms), and things also don’t exist even when many are certain they do (e.g., anthropomorphic God, aether, phlogiston). About what one can not speak, one should remain silent, and you can’t talk about something that is as logically vacuous as aggregate supply and demand.
Don’t say Aggregate Demand, say subsidies to investment of some kind, or more government spending, because that’s more meaningful, and then ask, should we be subsidizing these investments, or having more government spending? The indirect effects are so speculative you might as well ignore them, and just ask if the direct effects are worthwhile.