are the
common subjects of our thought and reasoning, and generally arise
from some principle of union among our simple ideas. These complex
ideas may be resolved into _relations_, _modes_, and
_substances_."--(_Ibid._)

In the next section, which is devoted to _Relations_, they are spoken of
as qualities "by which two ideas are connected together in the
imagination," or "which make objects admit of comparison," and seven
kinds of relation are enumerated, namely, _resemblance_, _identity_,
_space and time_, _quantity or number_, _degrees of quality_,
_contrariety_, and _cause and effect_.

To the reader of Hume, whose conceptions are usually so clear, definite,
and consistent, it is as unsatisfactory as it is surprising to meet with
so much questionable and obscure phraseology in a small space. One and
the same thing, for example, resemblance, is first called a "quality of
an idea," and secondly a "complex idea." Surely it cannot be both. Ideas
which have the qualities of "resemblance, contiguity, and cause and
effect," are said to "attract one another" (save the mark!), and so
become associated; though, in a subsequent part of the _Treatise_,
Hume's great effort is to prove that the relation of cause and effect is
a particular case of the process of association; that is to say, is a
result of the process of which it is supposed to be the cause. Moreover,
since, as Hume is never weary of reminding his readers, there is nothing
in ideas save copies of impressions, the qualities of resemblance,
contiguity, and so on, in the idea, must have existed in the impression
of which that idea is a copy; and therefore they must be either
sensations or emotions--from both of which classes they are excluded.

In fact, in one place, Hume himself has an insight into the real nature
of relations. Speaking of equality, in the sense of a relation of
quantity, he says--

"Since equality is a relation, it is not, strictly speaking, a
property in the figures themselves, but arises merely from the
comparison which the mind makes between them."--(I. p. 70.)

That is to say, when two impressions of equal figures are present, there
arises in the mind a _tertium quid_, which is the perception of
equality. On his own principles, Hume should therefore have placed this
"perception" among the ideas of reflection. However, as we have seen, he
expressly excludes everything but the emotions and the passions from