One major difference between Keynes and the Classical economists was that while the latter saw "full employment" as the normal state of affairs with a free-market economy (except for short periods of adjustment), Keynes saw the possibility of persistent aggregate-demand failure causing unemployment rates to exceed those corresponding to full employment. Put differently, while Classical economists saw all unemployment as "voluntary", Keynes saw the possibility that involuntary unemployment can exist when the demand for final products is low compared to potential output.