iv.) resemblance, contiguity in time and space, and cause and effect, are said to be the "uniting principles among ideas," "the bond of union" or "associating quality by which one idea naturally introduces another." Hume affirms that-- "These qualities produce an association among ideas, and upon the appearance of one idea naturally introduce another." They are "the principles of union or cohesion among our simple ideas, and, in the imagination, supply the place of that inseparable connection by which they are united in our memory.

Here is a kind of _attraction_, which, in the mental world, will be found to have as extraordinary effects as in the natural, and to show itself in as many and as various forms.

Its effects are everywhere conspicuous; but, as to its causes they are mostly unknown, and must be resolved into _original_ qualities of human nature, which I pretend not to explain."-- (I.p.

29.) And at the end of this section Hume goes on to say-- "Amongst the effects of this union or association of ideas, there are none more remarkable than those complex ideas which 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.