ve acquired a connexion in our
thought, and give rise to this inference, by which they become
proofs of each other's existence; a conclusion which is somewhat
extraordinary, but which seems founded on sufficient
evidence."--(IV. pp. 87-89.)

In the fifteenth section of the third part of the _Treatise_, under the
head of the _Rules by which to Judge of Causes and Effects_, Hume gives
a sketch of the method of allocating effects to their causes, upon
which, so far as I am aware, no improvement was made down to the time of
the publication of Mill's _Logic_. Of Mill's four methods, that of
_agreement_ is indicated in the following passage:--

" ... where several different objects produce the same effect, it
must be by means of some quality which we discover to be common
amongst them. For as like effects imply like causes, we must always
ascribe the causation to the circumstance wherein we discover the
resemblance."--(I. p. 229.)

Next, the foundation of the _method of difference_ is stated:--

"The difference in the effects of two resembling objects must
proceed from that particular in which they differ. For, as like
causes always produce like effects, when in any instance we find
our expectation to be disappointed, we must conclude that this
irregularity proceeds from some difference in the causes."--(I. p.
230.)

In the succeeding paragraph the _method of concomitant variations_ is
foreshadowed.

"When any object increases or diminishes with the increase or
diminution of the cause, 'tis to be regarded as a compounded
effect, derived from the union of the several different effects
which arise from the several different parts of the cause. The
absence or presence of one part of the cause is here supposed to be
always attended with the absence or presence of a proportionable
part of the effect. This constant conjunction sufficiently proves
that the one part is the cause of the other. We must, however,
beware not to draw such a conclusion from a few experiments."--(I.
p. 230.)

Lastly, the following rule, though awkwardly stated, contains a
suggestion of the _method of residues_:--

" ... an object which exists for any time in its full perfection
without any effect, is not the sole cause of that effect, but
requires to be assisted by some other principle, which may forward