Humeanism
Humeanism refers to the philosophy of David Hume and to the tradition of thought inspired by him. Hume was an influential eighteenth century Scottish philosopher well known for his empirical approach, which he applied to various fields in philosophy. In the philosophy of science, he is notable for developing the regularity theory of causation, which in its strongest form states that causation is nothing but constant conjunction of certain types of events without any underlying forces responsible for this regularity of conjunction. This is closely connected to his metaphysical thesis that there are no necessary connections between distinct entities. The Humean theory of action defines actions as bodily behavior caused by mental states and processes without the need to refer to an agent responsible for this. The slogan of Hume's theory of practical reason is that "reason is...the slave of the passions". It restricts the sphere of practical reason to instrumental rationality concerning which means to employ to achieve a given end. But it denies reason a direct role regarding which ends to follow. Central to Hume's position in metaethics is the is-ought distinction. It states that is-statements, which concern facts about the natural world, do not imply ought-statements, which are moral or evaluative claims about what should be done or what has value. In philosophy of mind, Hume is well known for his development of the bundle theory of the self. It states that the self is to be understood as a bundle of mental states and not as a substance acting as the bearer of these states, as is the traditional conception. Many of these positions were initially motivated by Hume's empirical outlook. It emphasizes the need to ground one's theories in experience and faults opposing theories for failing to do so. But many philosophers within the Humean tradition have gone beyond these methodological restrictions and have drawn various metaphysical conclusions from Hume's ideas.
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: "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: "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.
Theory of action
try to determine what actions are, specifically their essential features. One important feature of actions, which sets them apart from mere behavior, is that they are intentional or guided "under an idea". On this issue, Hume's analysis of action emphasizes the role of psychological faculties and states, like reasoning, sensation, memory, and passion. It is characteristic of his outlook that it manages to define action without reference to an agent. Agency arises instead from psychological states and processes like beliefs, desires and deliberation. Some actions are initiated upon concluding an explicit deliberation on which course of action to take. But for many other actions, this is not the case. Hume infers from this that "acts of the will" are not a necessary requirement for actions.The most prominent philosopher of action in the Humean tradition is Donald Davidson. Following Hume in defining actions without reference to an agent, he holds that actions are bodily movements that are caused by intentions. The intentions themselves are explained in terms of beliefs and desires. For example, the action of flipping a light switch rests, on the one hand, on the agent's belief that this bodily movement would turn on the light and, on the other hand, on the desire to have light. According to Davidson, it is not just the bodily behavior that counts as the action but also the consequences that follow from it. So the movement of the finger flipping the switch is part of the action as well as the electrons moving through the wire and the light bulb turning on. Some consequences are included in the action even though the agent did not intend them to happen. It is sufficient that what the agent does "can be described under an aspect that makes it intentional". So, for example, if flipping the light switch alerts the burglar then alerting the burglar is part of the agent's actions.
One important objection to Davidson's and similar Humean theories focuses on the central role assigned to causation in defining action as bodily behavior caused by intention. The problem has been referred to as wayward or deviant causal chains. A causal chain is wayward if the intention caused its goal to realize but in a very unusual way that was not intended, e.g. because the skills of the agent are not exercised in the way planned. For example, a rock climber forms the intention to kill the climber below him by letting go of the rope. A wayward causal chain would be that, instead of opening the holding hand intentionally, the intention makes the first climber so nervous that the rope slips through his hand and thus leads to the other climber's death. Davidson addresses this issue by excluding cases of wayward causation from his account since they are not examples of intentional behavior in the strict sense. So bodily behavior only constitutes an action if it was caused by intentions in the right way. But this response has been criticized because of its vagueness since spelling out what "right way" means has proved rather difficult.