An Enquiry Concerning Human Understanding | Study Guide

David Hume

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An Enquiry Concerning Human Understanding | Main Ideas


Impressions and Ideas


Impressions are sensations. These are the sole source of all ideas and all knowledge. Hume argues that any legitimate idea must be traceable back to an original impression. Impressions are internal or external. Internal impressions include emotions and various sensations, while external impressions involve the organs of sense: sight, sound, touch, smell, and taste.

Hume's investigation into the mind is rigorously directed by the origin of all ideas, namely impressions. While he does acknowledge that human reason has the ability to consider nonempirical ideas, these are the demonstrable ideas of mathematics. Any attempt to extend beyond these bounds results, as he points out in Chapter 12, in "sophistry and illusion." One question that arises is why, given his thoroughgoing empiricism, Hume accepts relations of ideas, which includes mathematics, as meaningful. Again, for Hume, meaningful ideas are those that can be traced back to an original sense impression, which should preclude, therefore, nonempirical ideas.

One explanation involves digging a bit deeper into the relationship between impressions and ideas. On the one hand, Hume presents ideas as if they are parasitic on impressions; he evaluates thoughts, for example, as always "inferior to the dullest sensation." This suggests that, in the order of what is real, sense impressions, as antecedent to ideas, are superior. On the other hand, both are "perceptions of the mind," which in Section 2 Hume divides "into two classes or species" whose difference is in the "degrees of [their] force and vivacity." In this sense, ideas are clearly not different in kind from impressions but are explicitly different in degree. Ideas are residual impressions. This connection is how the mind, for example, can generate the idea of a golden mountain. No such thing exists, but the constituent ideas can be traced back to original impressions.

With this connection between impressions and ideas in mind, Hume's respect for relations of ideas—nonempirical ideas—can be clarified. Hume does not think there is an empirical origin of mathematical or logical ideas. There is also no impression of relations between them. Nevertheless, when mathematical or logical ideas are compared to each other, a clear relation emerges. This is not the case with comparisons made between sense impressions and resultant matters of fact. So, for example, calculating the dimensions of a room involves relying on the mind in a different way than does thinking about the differences and similarities between sunrise and sunset. There is, consequently, no a priori validation of matters of fact—no process independent of, or prior to, experience that could explain a supposed connection between one matter of fact and another.


An idea is a copy of an impression. It is always less real, less vivid, and less lively than the impression of which it is a copy. Any legitimate idea must be traced back to an original sense impression. There are, however, ideas for which no corresponding impression can be found. Such ideas include causation.

A major challenge Hume must meet is how to account for apparently nonempirical ideas that are not obviously fictitious or illusory. "There are," he points out in Section 7, "no ideas, which occur in metaphysics, more obscure and uncertain, than those of power, force, energy or necessary connection." The movement of one object—a billiard ball, for example—precedes the movement of a second ball. In the event itself, there is no impression of a connection between them, let alone one that is necessary. There is no impression of "power, force, [or] energy" in the first billiard ball that would provide empirical grounds for asserting such a connection. Nevertheless, these ideas exert considerable influence over virtually all aspects of human life.

Hume meets this challenge on empirical terms. The fact that such ideas exist does not imply that empiricism is metaphysically or epistemologically flawed. Instead, it is traditional accounts of the idea itself that are flawed. These accounts root the ideas either in an empirical or nonempirical origin. Hume argues, instead, that the source is psychological: the idea of causation is a custom or habit of the mind.

Cause and Effect

Hume's central argument in An Enquiry Concerning Human Understanding concerns the epistemological status of causation. His conclusion serves as a standard by which a variety of ideas are evaluated, from the idea that God exists to the idea that one is completely free to act. According to Hume, the idea of causation is a belief that events happen in a certain way. It is a psychological state that naturally arises from repeated similar experiences. It cannot, however, be rationally justified.

Ideas are justified by two modes of reasoning: experiential and nonexperiential reasoning. The first is inductive, and the second is deductive. Inductive reasoning typically proceeds from particular observations to general conclusions, that is, about what can be inferred from the observations taken together. An analogical inference is another typical type of inductive inference. In this case, from a series of observations about similarities between two or more objects, an inference is made about one of them. Hume argues that inductive reasoning that produces judgments he calls "matters of fact" cannot justify the idea of causation. That's because the idea of a causal relation between objects and events itself is the foundation of experiential reasoning. It is, as Hume says, "by means of that relation alone we can go beyond the evidence of our memory and senses." So, although experience is the occasion for the idea, it is not what justifies it.

Nonexperiential, or deductive, reasoning fares no better in producing a justification of the idea of causation. That's because "relations of ideas" reflect necessary connections between ideas. More specifically, one need only analyze the ideas in order to derive a relation between them. As Hume points out, only mathematical and logical ideas are so related. Experiential ideas, on the other hand, cannot be connected in this way, no matter how hard one may try. "Adam, though his rational faculties be supposed, at the very first, entirely perfect, could not have inferred from the fluidity, and transparency of water, that it would suffocate him, or from the light and warmth of fire, that it would consume him." This is because objects and events identified as causes do not contain their effects—effects cannot be extracted from causes by mere analysis. Hume concludes in Part 1 of Section 5 that "custom," not reasoning, is the source of the idea of causation.

Propensity of the Mind

Hume accounts for the idea of causation in terms of a "propensity [that] is the effect of Custom." The psychological certainty that one event precipitates another is Hume's answer to the question, "Where does my idea of causation originate?" According to Hume, causation is not discernible in nature and is not an a priori idea. Instead, the mind develops an expectation that one event or object will obtain whenever it is presented with one that has always been conjoined with it. It is, then, the "constant conjunction" of objects and events that provides the mind the occasion for producing the idea of causation.

"Custom, then, is the great guide of human life. It is that principle alone, which renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past." By this quote, Hume makes clear his optimism about the idea of causation. Even if the results of his empirical analysis limit the scope of epistemological claims about causation, Hume is not interested in eliminating its use. Navigating life would be practically impossible for human beings without it.

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