Aristotle, Nicomachean Ethics
H. Rackham, Ed.

Now money serves us as a guarantee of exchange in the future: supposing we need nothing at the moment, it ensures that exchange shall be possible when a need arises, for it meets the requirement of something we can produce in payment so as to obtain the thing we need. Money, it is true, is liable to the same fluctuation of demand as other commodities, for its purchasing power varies at different times; but it tends to be comparatively constant. Hence the proper thing is for all commodities to have their prices fixed; this will ensure that exchange, and consequently association, shall always be possible. Money then serves as a measure which makes things commensurable and so reduces them to equality. If there were no exchange there would be no association, and there can be no exchange without equality, and no equality without commensurability. Though therefore it is impossible for things so different to become commensurable in the strict sense, our demand furnishes a sufficiently accurate common measure for practical purposes.