I don't think Marshall reversed the axes. The first thing I do with demand is to show that, for a rational consumer, demand is simply a (marginal) willingness to pay schedule - which of course relates the (dependent) willingness to pay to the (independent) quantity. The next lesson is that market demand is simply the horizontal summation of these individual curves. The individual producer's supply curve, likewise, relates the (dependent)marginal cost to the (independent) quantity produced (with the usual qualifications) ; while market supply is the horizontal aggregation of these. Reading the demand curve "vertically'," as it were, may seem less natural than reading it 'horizontally" (for any P, how much will consumers demand?) but the former is related to the latter as explanans to explanandum. As far as I'm concerned, Alfred was right ! Kevin Quinn