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Humeanism

Philosophy and tradition inspired by David Hume

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Through Causality and Essence

Causality and necessity

Causality is usually understood as a relation between two events where the earlier event is responsible for bringing about or necessitating the later event. Hume's account of causality has been influential. His first question is how to categorize causal relations. On his view, they belong either to relations of ideas or matters of fact. This distinction is referred to as Hume's fork. Relations of ideas involve necessary connections that are knowable a priori independently of experience. Matters of fact, on the other hand, concern contingent propositions about the world knowable only a posteriori through perception and memory. Causal relations fall under the category of matters of facts, according to Hume, since it is conceivable that they do not obtain, which would not be the case if they were necessary. For Hume's empiricist outlook, this means that causal relations should be studied by attending to sensory experience. The problem with this is that the causal relation itself is never given directly in perception. Through visual perception, for example, we can know that a stone was first thrown in the direction of a window and that subsequently, the window broke, but we do not directly see that the throwing caused the breaking. This leads to Hume's skeptical conclusion: that, strictly speaking, we do not know that a causal relation was involved. Instead, we just assume it based on earlier experiences that had very similar chains of events as their contents. This results in a habit of expecting the later event given the impression of the earlier one. On the metaphysical level, this conclusion has often been interpreted as the thesis that causation is nothing but constant conjunction of certain types of events. This is sometimes termed the "simple regularity theory of causation".

A closely related metaphysical thesis is known as Hume's dictum: "[t]here is no object, which implies the existence of any other if we consider these objects in themselves". Jessica Wilson provides the following contemporary formulation: "[t]here are no metaphysically necessary connections between wholly distinct, intrinsically typed, entities". Hume's intuition motivating this thesis is that while experience presents us with certain ideas of various objects, it might as well have presented us with very different ideas. So when I perceive a bird on a tree, I might as well have perceived a bird without a tree or a tree without a bird. This is so because their essences do not depend upon one another. Followers and interpreters of Hume have sometimes used Hume's dictum as the metaphysical foundation of Hume's theory of causation. On this view, there cannot be any causal relation in a robust sense since this would involve one event necessitating another event, the possibility of which is denied by Hume's dictum.

Hume's dictum has been employed in various arguments in contemporary metaphysics. It can be used, for example, as an argument against nomological necessitarianism, the view that the laws of nature are necessary, i.e. are the same in all possible worlds. To see how this might work, consider the case of salt being thrown into a cup of water and subsequently dissolving. This can be described as a series of two events, a throwing-event and a dissolving-event. Necessitarians hold that all possible worlds with the throwing-event also contain a subsequent dissolving-event. But the two events are distinct entities, so according to Hume's dictum, it is possible to have one event without the other. David Lewis follows this line of thought in formulating his principle of recombination: "anything can coexist with anything else, at least provided they occupy distinct spatiotemporal positions. Likewise, anything can fail to coexist with anything else". Combined with the assumption that reality consists on the most fundamental level of nothing but a spatio-temporal distribution of local natural properties, this thesis is known as "Humean supervenience". It states that laws of nature and causal relations merely supervene on this distribution of local natural properties. An even wider application is to use Hume's dictum as the foundational principle determining which propositions or worlds are possible and which are impossible based on the notion of recombination.

Not all interpreters agree that the reductive metaphysical outlook on causation of the Humean tradition presented in the last paragraphs actually reflects Hume's own position. Some argue against the metaphysical aspect, instead claiming that Hume's view concerning causality remained within the field of epistemology as a skeptical position on the possibility of knowing about causal relations. Others, sometimes referred to as the "New Hume tradition", reject the reductive aspect by holding that Hume was, despite his skeptical outlook, a robust realist about causation.

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