But the difference between Smith and Ricardo is this: for the first, labour, because it is analysable into days of subsistence, can be used as a unit common to all other merchandise (including even the commodities necessary to subsistence themselves); for the second, the quantity of labour makes it possible to determine the value of a thing, not only because the thing is representable in units of work, but first and foremost because labour as a producing activity is 'the source of all value'.