Regarding Foldvary on how AD is derived:
Correct me if I am wrong, but I seem to dimly recall, when being taught 
Macro a long time ago by none other than my mentor Murray Rothbard (no 
friend of Maynard), that if we let the price level vary, thus shifting IS up 
and down, we obtain a "map of notional equilibrium points in the goods 
market", and thus, as he put it, "when the points are connected", a 
"primitive form" of what can be termed, an  "Aggregate Demand schedule" 
(remember that IS is a construct itself...)

I seem to also recall that this was the basis for Barro's critique of the 
whole IS-LM-AD-AS framework .....But I defer to A.K. Dutt, who has written 
the history of AD-AS to confirm this, and hope if he is reading the messages 
on the list, he will tell us the true story...

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Warren Young