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 this group.
It is necessary therefore to amend Hume's primary "geography of the mind" by the excision of one territory and the addition of another; and the elementary states of consciousness will stand thus:—
A. Impressions.
a. Sensations of
a. Smell.
b. Taste.