"Keynes turned the question back again. He started thinking in Ricardo’s terms: output as a whole and why worry about a cup of tea? When you are thinking about output as a whole, relative prices come out in the wash — including the relative price of money and labour. The price level comes into the argument, but it comes in as a complication, not as the main point. If you have had some practice on Ricardo’s bicycle you do not need to stop and ask yourself what to do in a case like that, you just do it. You assume away the complication till you have got the main problem worked out. So Keynes began by getting money prices out of the way. Marshall’s cup of tea dissolved into thin air. But if you cannot use money, what unit of value do you take? A man hour of labour time. It is the most handy and sensible measure of value, so naturally you take it. You do not have to prove anything, you just do it."