Causality

In his ground-breaking A Treatise of Human Nature (1739–1740), David Hume made the scientific hunt for causes possible, by freeing the concept of causality from the metaphysical chains that his predecessors had used to pin it down. For Hume, causality, as it is in the world, is a regular succession of event-types: one thing invariably following another. His famous first definition of causality runs as follows: "We may define a CAUSE to be 'An object precedent and contiguous to another, and where all the objects resembling the former are plac'd in like relations of precedency and contiguity to those objects, that resemble the latter'" (1978 ed., p. 170).

Taking a cue from Malebranche, Hume argued that there was no perception of the supposed necessary connection between the cause and the effect. When a sequence of events that is considered causal is observed—for example, two billiard balls hitting each other and flying apart—there are impressions of the two balls, of their motions, of their collision, and of their flying apart, but there is no impression of any alleged necessity by which the cause brings about the effect. Hume went one step further. He found worthless his predecessors' appeals to the power of God to cause things to happen, since, as he said, such claims give us "no insight into the nature of this power or connection" (p. 249). So, Hume secularized completely the notion of causality. He also found inadequate, because circular, his predecessors' attempts to explain the link between causes and effects in terms of powers, active forces, and so on. As he put it: "[T]he terms efficacy, agency, power, force, energy, necessity, connexion, and productive quality, are all nearly synonymous; and therefore 'tis an absurdity to employ any of them in defining the rest" (p. 157).

Yet Hume faced a puzzle. According to his empiricist theory of ideas, there are no ideas in the mind unless there were prior impressions (perceptions). He did, however, recognize that the concept of causality involved the idea of necessary connection. Where does this idea come from, if there is no perception of necessity in causal sequences? Hume argued that the source of this idea is the perception of "a new relation betwixt cause and effect": a "constant conjunction" such that "like objects have always been plac'd in like relations of contiguity and succession" (p. 88). The perception of this constant conjunction leads the mind to form a certain habit or custom: to make a "customary transition" from cause to effect. It is this felt determination of the mind that affords us the idea of necessity.

So instead of ascribing the idea of necessity to a feature of the natural world, Hume took it to arise from within the human mind, when the latter is conditioned by the observation of a regularity in nature to form an expectation of the effect, when the cause is present. Indeed, Hume offered a second definition of causality: "A CAUSE is 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" (p. 170). Hume thought that he had unpacked the "essence of necessity": it "is something that exists in the mind, not in the objects" (p. 165). He claimed that the supposed objective necessity in nature is spread by the mind onto the world. Hume can be seen as offering an objective theory of causality in the world (since causation amounts to regular succession), which was however accompanied by a mind-dependent view of necessity. This dual aspect of Hume's account of causality is reflected in his two definitions.

Being an empiricist, Hume argued that all causal knowledge stems from experience. He revolted against the traditional view that the necessity that links cause and effect is the same as the logical necessity of a demonstrative argument. He argued that there can be no a priori demonstration of any causal connection, since the cause can be conceived without its effect and conversely. His far-reaching observation was that the alleged necessity of causal connection cannot be proved empirically either. As he famously argued, any attempt to show, based on experience, that a regularity that has held in the past will or must continue to hold in the future will be circular and question-begging. It will presuppose a principle of uniformity of nature. But this principle is not a priori true. Nor can it be proved empirically without circularity. For any attempt to prove it empirically will have to assume what needs to be proved—namely, that since nature has been uniform in the past it will or must continue to be uniform in the future. This Humean challenge to any attempt to establish the necessity of causal connections on empirical grounds has become known as his skepticism about induction. But it should be noted that Hume never doubted that people think and reason inductively. He just took this to be a fundamental psychological fact about human beings that cannot be accommodated within the confines of the traditional conception of Reason. Indeed, Hume went on to describe in detail some basic "rules by which to judge of causes and effects" (p. 173).