Three concepts prove integral to understanding causation as described by Hume:
contiguity, priority of succession, and “necessary connexion.”
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Hume observes that “tho’ distant
objects may sometimes seem productive of each other, they are commonly found upon
examination to be link’d by a chain of causes, which are contiguous among themselves, and to
the distant objects […] we may therefore consider the relation of CONTIGUITY as essential to that
of causation” (1.3.2.6). In this instance, Hume acknowledges the multiplicity of meaning
contained within “contiguity” and the appropriateness of these multiple meanings to his concept
of causation. Occasionally, contiguity implies proximity so close that a cause or series of causes
makes physical contact with the effects or objects experiencing the effects; for example, one
witnesses the cause-and-effect relationship when billiard balls move after striking each other. At
other times, contiguity connotes a relation that, while intimate, does not necessarily have to
maintain physical contact. Such a relationship exists between rain and wet grass; while one
might not see the rain hitting individual blades of grass, the undeniable link between the two
remains. Whether one billiard ball strikes another and sets the second ball into motion, or rain
causes grass to become wet, though, the proximity of the causes to the effects remains the crux
of the contiguity which Hume considers vital to causation.
Hume’s second key concept for causation, priority of succession, appears
straightforward: a cause must always precede its effect. Hume arrives at the necessity of priority
of succession through “a kind of inference or reasoning,” explaining that “if any cause may be
perfectly co-temporary with its effect, ‘tis certain […] that they must all of them be so […] The
consequence of this wou’d be no less than the destruction of that succession of causes, which we
observe in the world; and indeed, the utter annihilation of time” (1.3.2.7). Although Hume offers
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The spelling of “necessary connexion” will be modernized throughout the remainder of this
thesis, except when quoting directly from Hume’s Treatise of Human Nature.