An Enquiry Concerning Human Understanding | Study Guide

David Hume

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An Enquiry Concerning Human Understanding | Section 6 : Of Probability | Summary



Having argued for the view that the idea of causation is a belief rather than an item of knowledge, Hume returns to an analysis of inductive, or probabilistic, reasoning. Having already asserted the significance of custom or habit as "the great guide of human life," Hume here discusses probabilistic reasoning and judgments in terms of confidence. An increase in the probability of an event correlates with an increase in confidence in that event occurring. As he puts it, the higher the probability is, the greater is the "belief or expectation of the event [being] more steady and secure." A decrease correlates accordingly.

Hume discusses probability in connection with chance, a concept he associates with ignorance. Chance is not a feature of the world but instead is what one asserts when one lacks the belief associated with a high level of confidence that an event will occur. As Hume puts it, "the very nature of chance" consists in an event happening or not happening "as alike probable," or "entirely equal." It is not the case that the future is undetermined, which is what chance suggests, but rather that one does not feel the "reliance or security, which constitutes the nature of belief and opinion."

The more "uniform and constant" is a series of events, such as fire and heat, the more confident one is about the causal connection between the two. Irregularities, on the other hand, make determining the effect more difficult. This does not mean, however, that there is no relation to be found. Instead, Hume asserts, one "must assign to each of [the effects] a particular weight and authority, in proportion as we have found it to be more or less frequent."


In this brief section, Hume makes some subtle distinctions in his existing theory of the mind. The intensity of a belief or conviction is proportional to the collective number of a certain type of events that precede an outcome. So, for example, the belief that the sun will rise tomorrow is so strong that it is beyond doubt, even if it is not logically guaranteed. In other words, the belief is on the side of the sun rising tomorrow rather than on the side of the sun not rising tomorrow. This is because the collective number of experiences is on the side of the belief that it will rise.

The belief about the sun rising is not offset by any experiences to the contrary, but there are plenty of such examples to be had. Hume's discussion of probability accounts for a range of intensities in a belief. Hume mentions cases in which the consistency, regularity, or uniformity of events is lacking. For example, rhubarb does not always purge certain ailments, and opium does not always act as a soporific. These sorts of cases are where probability is most useful. As Hume points out, "Though we give the preference to that which has been found most usual ... we must not overlook the other effects, but must assign to each of them a particular weight and authority, in proportion as we have found it to be more or less frequent."

The frequency of an event leads the mind to expect that event to happen, given the relevant circumstances. This is why, for example, one tends to believe that a coin toss is more likely to land on heads—or tails—because it has done so for, say, the past ten tosses. This belief occurs despite the fact that the probability is always fifty-fifty—it does not increase with the number of times the coin lands on a specific side. The fact that one makes this error reflects the strength of the sort of belief Hume is talking about here.

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