I advance what might be thought a paradoxical thesis: that the central topic of Hume’s long discussions “Of the Idea of Necessary Connexion” is not, in fact, the idea of necessary connexion. However it is not as paradoxical as it first appears, for I shall claim that the “idea” whose origin Hume seeks is, in a sense, an idea-type of which the specific idea of necessary connexion is but one instance. Various lines of evidence support this claim, but my main argument will rest on its ability to solve four puzzles in Hume’s text, which are otherwise hard to explain away. These are: (S) the synonymy puzzle, posed by Hume’s apparently reckless assertion that “efficacy”, “agency”, “power”, “force”, “energy”, “necessity”, “connexion”, and “productive quality” are all virtual synonyms; (C) the complexity puzzle, that Hume seems to ignore the possibility that his target idea might be complex rather than simple; (V) the vulgar problem, which arises from Hume’s acknowledgement that the vulgar believe in “chancy” causes, even though he takes the very concept of causation to involve necessity; and (P) the probability problem, of how an allegedly simply idea whose central core involves inexorable necessity could possibly provide a basis for probability. The paper ends by drawing further support from an analysis of Hume’s two sections “Of the idea of necessary connexion”, showing that his use of the various relevant terms makes good sense on the thesis proposed, thus corroborating the arguments presented.