why the natural level of production is constant regardless of the natural interest rate changes is really because its the level of production that the firms uses its resource the most efficiently?I do think price level, nominal rate of interest do not affect the natural level of output, so what factors affect the natural level of output? please illustrate with diagrams if possible, thanks a lot!

I-'lg. 4.9 The edict of optimism on the natural rate of Interest f In to 1,, but because of the increase in the interest rate. investment will increase only to I]