Money:

It is therefore necessary that all commodities shall be measured by some one standard, as was said before. And this standard is in reality demand, which is what holds everything together, since if men cease to have wants or if their wants alter, exchange will go on no longer, or will be on different lines. But demand has come to be conventionally represented by money; this is why money is called nomisma （customary currency）, because it does not exist by nature but by custom （nomos）, and can be altered and rendered useless7 at will. (Aristot. Nic. Eth. 5.5)