has been experienced at different periods to check gold from rising to £5 4s. per ounce, and we now notice that gold is quoted at £3 17s. 9d. per ounce?... The expression _pound_ has reference to value, but not a fixed standard value.... The term pound is the _ideal unit_.... Labour is the parent of cost and gives the relative value to gold or iron. _Whatever denomination of words are used to express the daily or weekly labour of a man_, such words express the cost of the commodity produced.”[55] In the last words the hazy conception of the ideal money measure melts away and its real meaning breaks through. The reckoning names of gold, pound sterling, shilling, etc., should be names for definite quantities of labor-time. Since labor-time constitutes the substance and the intrinsic measure of values, these names would then actually represent definite proportions of value. In other words, labor-time is maintained to be the true unit of measure of money. With this we leave the Birmingham school, but should add in passing that the doctrine of the ideal measure of money acquired new importance in the controversy over the question of the convertibility or non-convertibility of bank notes. If paper receives its name from gold or silver, then the convertibility of a note or its exchangeability for gold or silver remains an economic law, no matter what the civil law may be. Thus a Prussian paper thaler, although legally inconvertible, would immediately depreciate if it were worth less than a silver thaler in ordinary trade, i. e., if it were not practically convertible. The consistent advocates of inconvertible paper money in England, therefore, sought refuge in the ideal measure of money. If the reckoning names of money, £, s., etc., are names of certain quantities of atoms of value, of which a commodity absorbs or loses now more, now less in exchange for other commodities, then an English £5 note, e. g., is just as independent of its relation to gold as of that to iron and cotton. Since its title would no more imply its theoretical equality with a certain quantity of gold or any other commodity, the demand for its convertibility, i. e., for its practical equality with a definite quantity of a specified thing would be excluded by the very conception of the note.