Hump put all this rather more pointedly. We have 'impressions,' i. e. lively perceptions by sense. We have also 'ideas,' i. e. fainter images of these, but otherwise identical. An idea _should_ be a copy of an impression. If you cannot point out any such impression, you may be certain you are mistaken when you imagine you have any such idea. There is prevalent in the mental world a kind of association; a 'gentle force' connects ideas in our imagination according to certain relations they possess. This 'mind' or this 'imagination' is only a bundle or collection of impressions and ideas; but a collection which is continually and rapidly changing in its constituents, and in the scale of liveliness possessed by each constituent. When an idea is particularly fresh and forcible, it is a _belief_ or it is believed in: when faint, not so. Or, otherwise put, the object of an idea is _said_ to _exist_, when the idea itself is vividly _felt_[3]. Really there is no such thing as 'external existence' taken literally. 'Our universe is the universe of the imagination': all existence is for a consciousness.
Impressions arise in certain orders of sequence or co-existence. When two impressions frequently recur and always in the same order, the custom binds them so closely together, that, should one of them only be given as impression, we cannot help having an idea of the other, which, growing more vivid by the contagion of the contiguous impression, creates, or is, a belief in its reality. Between the perceptions as such, there is no connexion; they are distinct and independent existences. They only get a connexion through _our feeling; we feel_ a 'determination' of our thought to pass from one to another. The one impression has no power to produce the other; the one thing does not cause the other. 'We never have any impression that contains any power or efficacy[4],' Hence the power and necessity we attribute to the so-called causal agent and to the connexion are an illegitimate transference from our feeling, and a mistranslation of _our_ incapacity to resist the force of habitual association into a real bond between the two impressions _themselves,_ The necessity is in the mind--as a habit-caused compulsion--not in the objects.
