I've watched the former chief economist of Goldman Sachs argue with his right-hand man about whether the big drop in crude prices in 2014 was due to supply or demand. If they can't figure it out, what hope do the rest of us have?

In almost all cases, neither supply nor demand are observable. They change through time - but we don't have a good model for how they change. The curves are nonlinear, and there's feedback between changes in supply and changes in demand.

Finally, in macroeconomics, even the theory is incoherent. The Sonnenschein-Mantel-Debreu theorem says that the aggregate demand curve can take any shape - it need not be 'downward sloping'.