I do see your point. Think about it this way: In the short run all the production costs are fixed (e.g. nominal wages are set for one year periods typically). Now if the aggregate price level increases, profit margins of the companies improve, since their production costs stay the same but their sales prices are increasing, which causes them to increase their production levels (or think of it as new companies entering the market because they hear of those great profit margins). The more we are in the short run, the stronger will be this adjustment in output because production costs are “more” fixed. As we move towards the long run, our productions costs become more flexible (once you go beyond a year, you can re-negotiate wages), which means as the aggregate price level changes, our production costs also change. Imagine that aggregate prices increase by 5% completely surprisingly. Companies will have their workers on one year contracts and those cannot be re-adjusted until one year passes. So within that one year, profit margins for the company improve, it produces more, but after one year the workers demand higher wages (since have been observing a 5% wage decrease over the last year in real terms), and now the company’s profit margins are at back where they were before the increase in prices. And the more we go into the long run, the more will this re-negotiation process take away any short term gains from changes in prices. Thus in the very long run, any change in aggr. prices will be met with a corresponding change in production costs, and thereby render the change in prices without any effect.