When we talk of any particular sum of money, we sometimes mean nothing but
the metal pieces of which it is composed, and sometimes we include in our
meaning some obscure reference to the goods which can be had in exchange for
it, or to the power of purchasing which the possession of it conveys. Thus,
when we say that the circulating money of England has been computed at
eighteen millions, we mean only to express the amount of the metal pieces,
which some writers have computed, or rather have supposed, to circulate in
that country. But when we say that a man is worth fifty or a hundred pounds
a-year, we mean commonly to express, not only the amount of the metal pieces
which are annually paid to him, but the value of the goods which he can
annually purchase or consume; we mean commonly to assertain what is or ought
to be his way of living, or the quantity and quality of the necessaries and
conveniencies of life in which he can with propriety indulge himself.

When, by any particular sum of money, we mean not only to express the amount
of the metal pieces of which it is composed, but to include in its
signification some obscure reference to the goods which can be had in
exchange for them, the wealth or revenue which it in this case denotes, is
equal only to one of the two values which are thus intimated somewhat
ambiguously by the same word, and to the latter more properly than to the
former, to the money's worth more properly than to the money.

Thus, if a guinea be the weekly pension of a particular person, he can in
the course of the week purchase with it a certain quantity of subsistence,
conveniencies, and amusements. In proportion as this quantity is great or
small, so are his real riches, his real weekly revenue. His weekly revenue
is certainly not equal both to the guinea and to what can be purchased with
it, but only to one or other of those two equal values, and to the latter
more properly than to the former, to the guinea's worth rather than to the
guinea.