I think of Keynes as someone who worked in aggregates. He spends chapter 3 of The General Theory on outlining the relationship between the aggregate supply function and the aggregate demand function. To me, a price-level is just an aggregate across prices. It is a simplification for looking at the whole economy.
That is why I was surprised when I reread his take on price-levels in chapter 4.
"the well-known, but unavoidable, element of vagueness which admittedly attends the concept of the general price-level makes this term very unsatisfactory for the purposes of a causal analysis, which ought to be exact. Nevertheless these difficulties are rightly regarded as 'conundrums'. They are 'purely theoretical' in the sense that they never perplex, or indeed enter in any way into, business decisions and have no relevance to the causal sequence of economic events, which are clear-cut and determinate in spite of the quantitative indeterminacy of these concepts. It is natural, therefore, to conclude that they not only lack precision but are unnecessary." (emphasis added-Pg 32 of my pdf version)
Price-levels are not unnecessary for historical analysis. Instead, they are unnecessary or improper for any causal theory, which is Keynes's endeavour. If anyone one can elaborate on why this aggregation is unnecessary, but supply and demand aggregation is vital, I will be grateful.