The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume X. The Age of Johnson.
§ 7. The Problem of Causation
This is a simple statement of the central argument of Hume’s most famous discussion. The “powers” which Locke attributed to bodies must be denied—as Berkeley denied them. The consciousness of spiritual activity on which Berkeley relied is equally illusory on Hume’s principles.
If we admit Hume’s fundamental assumption about impressions and ideas, it is impossible to deny the general validity of this reasoning. Any assertion of a causal connection—the whole structure of natural science, therefore—is simply a misinterpretation of certain mental processes. At the outset, Hume himself had spoken of impressions as arising from “unknown causes”; and some expressions of the sort were necessary to give his theory a start and to carry the reader along with him; but they are really empty words. Experience is confined to impressions and ideas; causation is an attitude towards them produced by custom—by the mode of sequence of ideas; its applicability is only within the range of impressions or ideas; to talk of an impression as caused by something that is neither impression nor idea may have a very real meaning to any philosopher except Hume; but to Hume it cannot have any meaning at all.