Brief description: Classical economists believe that in the short run the aggregate supply curve may be upward sloping, but in the long run it will be vertical.

Detailed description: In the short run an increase in aggregate demand may lead to an increase in output, but there will also be an increase in the price level. As firms realise that there has not been a real increase in demand, they will once again reduce output and the economy will tend back to the full employment level of output.