–I admit to a fantasy about supply and demand curves, provoked by all those graphics like the one culled below from Wikipedia:
My fantasy is this: Imagine both demand and supply shifting downwards, with equilibrium price P* and quantity Q* shifting down with them. At some point, the two curves intersect the horizontal axis, producing three quantities, Qs < Q* < Qd.
Now, if you take a second look at the graphic, Qs is the quantity supplied even when price is zero. The equilibrium price, P*, becomes the price needed to move the quantity supplied from Qs to Q*. In this way, a portion of the quantity demanded is provided at no price because of, say, intrinsic motivation, or suppliers are confused, or everyone was just lucky. I’d like to think that is the good mess somewhere in every equilibrium analysis.
Frey, B.S. (1997). Not Just for the Money: An Economic Theory of Personal Motivation. Edward Elgar Publishing: Cheltenham, UK.