In strictness, indeed, it is impossible to find any common measure for things so extremely diverse; but our needs give a standard which is sufficiently accurate for practical purposes.

There must, then, be some one common symbol for this, and that a conventional symbol; so we call it money ( νόμισμα, νόμος ). Money makes all things commensurable, for all things are valued in money. For instance, let A stand for a house, B for ten minae, C for a bed; and let A = B÷2, taking a house to be worth or equal to five minae, and let C (the bed) = B÷10. We see at once, then, how many beds are equal to one house, viz. five.

It is evident that, before money came into use, all exchange must have been of this kind: it makes no difference whether you give five beds for a house, or the value of five beds.