BOOK V

And further, money is a kind of security to us in respect of exchange at some future time (supposing that one wants nothing now that we shall have it when we do): the theory of money being that whenever one brings it one can receive commodities in exchange: of course this too is liable to depreciation, for its purchasing power is not always the same, but still it is of a more permanent nature than the commodities it represents. And this is the reason why all things should have a price set upon them, because thus there may be exchange at any time, and if exchange then dealing. So money, like a measure, making all things commensurable equalises them: for if there was not exchange there would not have been dealing, nor exchange if there were not equality, nor equality if there were not the capacity of being commensurate: it is impossible that things so greatly different should be really commensurate, but we can approximate sufficiently for all practical purposes in reference to Demand. The common measure must be some