suggesting that it is really gold. Then he may propose to dip it in
vinegar; whilst we agree that, if it then turns green, it is copper and
not gold. On trying this experiment the metal does turn green; so that
we may put his argument in this way:--

_Whatever yellow metal turns green in vinegar is copper;
This yellow metal turns green in vinegar;
Therefore, this yellow metal is copper._

Such an argument is called proof by mediate inference; because one
cannot see directly that the yellow metal is copper; but it is admitted
that any yellow metal is copper that turns green in vinegar, and we are
shown that this yellow metal has that property.

Now, however, it may occur to us, that the liquid in which the metal was
dipped was not vinegar, or not pure vinegar, and that the greenness was
due to the impurity. Our friend must thereupon show by some means that
the vinegar was pure; and then his argument will be that, since nothing
but the vinegar came in contact with the metal, the greenness was due to
the vinegar; or, in other words, that contact with that vinegar was the
cause of the metal turning green.

Still, on second thoughts, we may suspect that we had formerly conceded
too much; we may reflect that, although it had often been shown that
copper turned green in vinegar, whilst gold did not, yet the same might
not always happen. May it not be, we might ask, that just at this
moment, and perhaps always for the future gold turns, and will turn
green in vinegar, whilst copper does not and never will again? He will
probably reply that this is to doubt the uniformity of causation: he may
hope that we are not serious: he may point out to us that in every