Because of continuous full employment in the economy, volume of output (Y) does not change in the long run. On the other hand, since money is the only medium of exchange in classical world, velocity of money remains constant. Classical economists insist that factors that determine the velocity are people’s habits to spend and to pay. These factors do not exhibit big changes, at least in the short run. In that case, when the supply of money (M) increases, for instance, 10 percent, since output (Y) and velocity (V) are assumed as constant, price level (P) must also increase 10 percent. As you noticed from this example, the rise in the supply of money directly increases aggregate expenditures and is reflected to nominal income with the same direction and the same rate. If you think of these changes, you realize of course that, the full increase in nominal income stems from increase in prices.