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.