While Pat ruled it out for reasons unexplained, most economists have treated policy changes, especially those of a discrete and unexpected variety (either arising in response to unexpected changes in exogenous events or changes in the composition and orientation of the policymakers) as exogenous in macroeconomics. Also, while asset bubbles can be viewed as endogenously arising from an economy, what pricks a bubble is usually some exogenous shock or event. However, the multiplier effects (if Pat is allowing such wicked things to exist) from the collapse of a bubble should probably be viewed as endogenous. Of course this whole question of exogeneity versus endogeneity has a certain arbitrary and artificial character to it. In formal terms it simply depends on how one defines the boundary of the system in question. What is generated by the system is endogenous; what comes from outside of it is exogenous. But what is inside or outside is arbitrarily defined ulitmately. The complexity economist William (Buz) Brock has infamously quipped that the only truly exogenous force in the economy is the sun, and he is not referring specifically to Jevonsian theories of the business cycle. OTOH, I am not sure that he is the originator of that quote. Barkley Rosser