Hume, David (1711–76)

DOI: 10.4324/9780415249126-DB040-2
Version: v2,  Published online: 2005
Retrieved January 23, 2019, from

4. Causal necessity

On the basis of his account of causal inference, Hume offers an explanation of the ‘necessary connexion’ of causal relations and provides two definitions of the term ‘cause’ (THN 1.3.14; EHU 7). It follows from the Copy Principle that, if we have an idea of necessary connection, that idea must be copied from some impression or impressions. When the mind first observes an event of one type followed by an event of another type, however, it never perceives any necessary connection between them; it is only after repeated experience that the mind pronounces them to be necessarily connected. Yet merely repeating the experience of the conjunction of two types of events cannot introduce any new impression into the mind from the objects themselves beyond what was perceptible on the first observation. The impression of necessary connection, from which the idea of necessary connection is copied, must therefore be an internal impression resulting from the respect in which the mind itself changes as the result of experience of constant conjunction. Accordingly, the impression of necessary connection is, Hume concludes, the impression of the mind’s own determination to make an inference from an impression or memory to a belief. This impression is often then projectively mislocated in or between the cause and effect themselves, in much the same way that non-spatial tastes, smells and sounds are erroneously located in bodies with which they are associated. Causal relations have a kind of necessity – an unthinkability of the opposite – that is grounded in the psychological difficulty of separating two types of events in the imagination after they have been constantly conjoined in perception, and in the impossibility of believing them actually to be separated. Because of the projective illusion by which the impression of necessary connection is mislocated in the objects, however, we often conflate this causal necessity, Hume explains, with the demonstrative necessity that results from intrinsic relations among ideas. The result, he claims, is philosophical confusion, in which we suppose that we can perceive a necessary connection, amounting to a demonstration, that is intrinsic to causes and effects themselves, and then become dissatisfied when we realize that, at least in some cases, we do not perceive such a connection after all. The dissatisfaction leads to disparate theories concerning how causal powers operate and to restrictions on the range of ‘genuine’ causal relations. The remedy for this confusion is to realize that we never make probable inferences as the result of perceiving a necessary connection between cause and effect, but rather, that we perceive the (internal impression of) necessary connection precisely because we are disposed to make the inference. Any two types of events are capable of standing in the causal relation to one another – ‘to consider the matter a priori, any thing may produce any thing’ (THN – and only experience can show that which are actually causally related.

Following his account of causal reasoning and causal necessity, Hume defines ‘cause’ in the Treatise both as ‘an object precedent and contiguous to another, and where all objects resembling the former are placed in like relations of precedency and contiguity to those objects, that resemble the latter’ and as ‘an object precedent and contiguous to another, and so united with it that the idea of the one determines the mind to form the idea of the other, and the impression of the one to form a more lively idea of the other’ (THN; see also EHU 7.33). It may well seem that these two definitions do not serve to pick out the same objects as causes; for objects can be constantly conjoined in fact without being observed to be so, and objects can be taken by observers of unrepresentative samples to be constantly conjoined that are not really so conjoined. In fact, however, both definitions are ambiguous, and in parallel ways. The first definition may be understood either in a subject-relative sense (concerned with what has been conjoined in the observation of a particular subject) or in an absolute sense (concerned with what is constantly conjoined at all time and places). The second definition may likewise be understood either in a subject-relative sense (concerned with what is a basis for association and inference in the mind of a particular subject) or an absolute sense (concerned with what is a basis for association and inference in an idealized human mind that has observed a representative sample of the conjunction in question and reasons in the ways that are philosophically approved). The two definitions then coincide on their subject-relative interpretations – the interpretations that Hume needs when discussing how causal relations constitute a principle of association in individual human minds, which is a matter of what a human mind will take to be causally related. The two definitions coincide again on their absolute interpretations – the interpretations that he needs when discussing which pairs of events are in fact causally related. Both definitions are intended to specify the class of ideas of pairs of events that are (either in a particular human mind or ideally) collected under the abstract idea of the relation of cause and effect. If cause-and-effect pairs have something else in common beyond what is captured by these two definitions, it is something inconceivable by the human mind. Interpreters differ on the question of Hume’s attitude towards the prospect of such an inconceivable ‘something more’ – some hold that he rejects it, while some hold that he allows it, and others hold that he assumes it. In any case, however, his conception of constant conjunction as necessary and sufficient for causation is one of the most influential ideas in the history of metaphysics.

Citing this article:
Garrett, Don. Causal necessity. Hume, David (1711–76), 2005, doi:10.4324/9780415249126-DB040-2. Routledge Encyclopedia of Philosophy, Taylor and Francis,
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