ABSTRACT
The cost of a basket of goods x when the prices are p is the sum px = ∑pixi, known by every shopper studying an account. There is no general possibility of expressing such as sum simply as a product PX, in effect, as if there were just one good, with quantity X and price P , P depending only on the prices of the goods and X on the quantities bought, say
P = e(p), X = f (x). Everyone should agreewith that. But the possibility appears to be taken for granted whenever terms like price level or price index are used, as they are constantly in macroeconomic discussions, and everyday with regard to the CPI. It can be wondered what sense there should be to the phenomenon.
