Hume’s epistemological orientation was that of a radical empiricism according to which all knowledge is derived from experience. Traditionally, this account has been contrasted with rationalist approaches which emphasize the role of reason (e.g., deductive inferences) and deny the claim that all human knowledge originates in experience. Rationalist thinkers such as Descartes (1596-1650) and Leibniz (1646-1716) proposed that there is a priori knowledge, that is, knowledge independent of experience from which we can derive new knowledge. For example, Descartes’ famous
“cogito, ergo sum” was claimed to be an a priori truth since it is gained through reason alone and not from experience. Some philosophers in the rationalist tradition, such as Leibniz, also allowed for the possibility of innate ideas.
Contrary to the rationalist position, the British empiricists John Locke (1632-1704), George Berkeley (1684-1753), and David Hume (1711-1776) claimed that the ultimate
PHILOSOPHICAL BACKGROUND 6 source of human knowledge is sense experience, not reason. The empiricists also denied
the existence of any innate knowledge (“innate ideas”), a position vividly expressed in Locke’s notion of the tabula rasa. In its most radical version, the empiricists’ position was that all our knowledge is a posteriori, that is, directly derived from experience.
Since all knowledge depends upon sense experience, the empiricists’ position implies that our causal knowledge must originate in experience, too. The question is then which of our experiences can give rise to knowledge of causal relations, one of the main issues Hume addressed in his writings.
Hume divides the mental realm into thoughts (“ideas”) and perceptions (“impressions”) which provide our mind with experience. According to Hume, even the most elaborated and abstract concepts (“complex ideas”) stem from, and are reducible to, atomic pieces of knowledge (“simple ideas”). These simple ideas, in turn, originate in the content of our experience (Hume’s so-called “copy thesis”). In his Treatise of Human Nature (1739/2000) Hume argues that “(…) all our simple ideas in the first appearance are deriv’d from simple impressions, which are correspondent to them, and which they exactly represent” (p. 9).
Embedded in Hume’s epistemological atomism is his analysis of causality.
According to Hume, causal knowledge is inductive, not deductive. For example, when we encounter a new object of which we have no knowledge, we cannot discover its causal history or its causal powers deductively. Thus, we are not capable of determining an object’s causes or effects by reason alone. Therefore, Hume concluded, knowledge of causal relations must be derived from experience. The problem he then faced was that the sensory input, our ultimate source of knowledge, does not contain any direct causal knowledge. Even though every event is a cause or an effect (or both), there is no feature (“quality”) common to all events which are kinds of cause and effect: “And indeed there is nothing existent, either externally or internally, which is not to be consider’d either as cause or an effect; tho’ ‘tis plain there is no one quality, which universally belongs to all beings, and gives them a title to that denomination. The idea, then, of causation must be deriv’d from some relation among objects; and that relation we must now endeavour to discover.” (Hume, 1739/2000, p. 53, his italics).
Hume’s task then was to determine which experiences give rise to the idea of causation. He proposed that causal relations are characterized by three features which are contained in our perceptions and can serve as sensory input to the process of causal induction. First, events of cause and effect are contiguous in space and time. This
relation is that of spatio-temporal contiguity. Second, events of cause and effect are temporally ordered since causes always precede their effects. This relation is that of temporal succession. However, two events might be contiguous and temporally ordered without being causally connected; therefore, contiguity and temporal priority are not sufficient to give rise to the idea of a causal relation. Hume argued that there is a third relation connecting causes and events and it is this relation that is essential to the idea of causation. Consistent with many other philosophers, Hume saw the impression of a
“necessary connexion” to be the fundamental feature of cause-effect relations. A necessary connection between two events implies that the cause necessitates the effect.
Since the cause is necessarily followed by the effect, observing the cause allows us to predict the presence of the effect event. It is in virtue of the acquaintance of this relation that we are able to transcend our past experience and make predictions about events not observed or not happened yet. However, contrary to many other philosophers before and after him, to Hume causal necessity is merely a construction of the human mind and must not be expected to exist outside our experience.
The problem is then to explain what gives rise to the impression of a necessary connection between two events. In analogy to the argument that we cannot determine an object’s causal powers by reason alone, Hume was convinced that we cannot logically prove the existence of a necessary connection between a cause and an effect. He proposed that it is the relation of constant conjunction from which we derive the idea of a necessary connection: “The idea of cause and effect is deriv’d from experience, which informs us, that such particular objects, in all past instances, have been constantly conjoin’d with each other.” (Hume, 1739/2000, p. 63). If we were only confronted with single episodes in which events occur together we would never induce a causal relation.
It is the repeated observation that events vary together which gives rise to the idea of a necessary connection and, eventually, generates the impression that these events are causally related. Hume did not claim that we can discover the exact nature of this connection from our sense experience but merely that we have the idea that there is such a connection. For example, we might infer from our experience that the moon is causally related to the tides even though we do not have specific knowledge of the exact nature of the underlying connection.
According to Hume, the impression of a causal relation implies that the idea of the cause event conveys the idea of the effect event. From the experienced constant conjunction of cause and effect we infer that upon the appearance of the cause the effect
PHILOSOPHICAL BACKGROUND 8 will follow, just as it did in the past. The crucial difference to the rationalist account is
Hume’s claim that such causal inferences are not a based on reasoning but on “the union of ideas”: “When the mind, therefore, passes from the idea or impression of one object to the idea or belief of another, it is not determin’d by reason, but by certain principles, which associate together the ideas of these objects, and unite them in imagination. (…) The inference, therefore, depends solely on the union of ideas” (Hume, 1739/2000, p.64). Thus, Hume denied that the presence of the effect is derived deductively from the existence of a necessary connection that binds together cause and effect. Instead, causal relations are inferred inductively according to associative learning principles; the inferences are merely “habit”, as he later stated in his An Enquiry Concerning Human Understanding (Hume, 1748/1993, p. 50). Since all inductive knowledge is fallible, he concluded, definite knowledge of causal relations lies beyond our reach.
To sum up, according to Hume’s empiricist approach the acquisition of causal knowledge is determined by spatio-temporal contiguity, temporal succession, and constant conjunction. When events are repeatedly perceived to be contiguous in space and time in several instances we will induce that they are causally related. The temporal information allows us to determine which event is the cause and which is the effect.
Since the information defined by these principles is contained in our sensory input, we have a well-defined account of data-driven causal induction even though our senses do not directly provide us with causal knowledge.