An Enquiry Concerning Human Understanding | Study Guide

David Hume

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An Enquiry Concerning Human Understanding | Section 7 : Of the Idea of Necessary Connection | Summary



Part 1

Hume has argued for the view that there is no knowledge of cause-and-effect relations, only belief. The intensity of the belief is proportional to the frequency of the observed relation between events claimed to be causes and events claimed to be effects. This belief is not, however, derived from a sense impression of that connection. In this section, Hume investigates whether or not that connection can, nevertheless, be understood as necessary. Is it the case, in other words, that causal connections are essentially natural laws? Is every effect contained in every cause?

Descriptions of cause-and-effect relations include "power, force, energy, or necessary connection." These are attributed to causal relations claimed for not only events and objects in nature, but also those claimed for metaphysics. For example, the will is claimed to be the cause of human action. Hume argues that such claims are even less plausible than those about natural events, since the supposed causes of the latter can at least be observed.

Hume argues that nature hides many of its powers from observation. So, although there is no belief about a causal connection prior to experience, it is also true that experience does not guarantee causal relations will be revealed. The motions in the human body are evidence of this claim. One does not observe the power whereby one voluntarily moves a limb anymore than one involuntarily breathes. Similarly, one can observe a succession of events: a billiard ball moves and makes contact with another billiard ball, and the second billiard ball moves. What is not observed is a causal connection—let alone a necessary one, which is to say, one that cannot be otherwise, given the relevant conditions. It is not legitimate to claim, therefore, that one event contains within it, so far as observation goes, an effect.

Hume offers three objections to claims that one can know, for example, that one has power over one's body. Each is based on the requirement that knowledge derives from sense impression. First, the presumed connection between the will or soul is entirely "mysterious." If there is to be a connection asserted between the soul and its causal power over the body, the soul should be at least observed—there should be some awareness of that power. Second, the will is claimed to have power over some bodily movements, such as the tongue, but not others, such as one's heartbeat. Moreover, some voluntary movements can become involuntary, as in the case of palsy. Third, what is claimed to be a voluntary movement involves a complex series of events, some of which are entirely unobservable. The observable movement of a limb involves muscles and nerves but, "perhaps," Hume claims, "something still more minute and more unknown." Hume concludes that the idea of power or necessary connection is not derived from a sense impression.

The same erroneous reasoning some thinkers employ in claiming to know a necessary connection exists between the soul and the body is marshaled for claims about God. Causes, they argue, "are in reality ... a volition of the Supreme Being." If, however, one does not know the necessary connection between natural events, one cannot rightly claim there is a God in whom resides the ultimate cause of all things.

Part 2

The idea of necessary connection cannot be traced to any original impression. It is, rather, like the belief in causation, the result of the mind having become accustomed or habituated to experiencing a constant conjunction of events.

Hume now sets out to define cause. He offers two definitions, the first of which focuses on objects. An object is a cause when it is followed by another "and where all the objects, similar to the first, are followed by objects similar to the second." The second definition focuses on the mental act whereby the appearance of one object "always conveys the thought to" the other object that follows it.


Thinkers such as Descartes have argued that causes contain their effects, in much the same way that a valid deductive argument's premises contain its conclusion. For example, the following argument's form is truth preserving, so that, even if one or both of the premises are false, the form forces the conclusion:

Logically Correct

Form of Logically Correct Reasoning

Logically Correct and True Premises

All cats are dogs.
All dogs are whales.
All cats are whales.

All C are D
All D are W
All C are W

All apples are fruits.

All fruits are foods containing Vitamin C.
All apples are foods containing Vitamin C.

Descartes uses a causal argument, for example, to infer God's existence from the idea of God. His reasoning is as follows: He has an idea of a supremely perfect being. The cause of the idea has to be at least as real as its effect, that is, of the idea itself. But Descartes is not perfect, so he cannot be the cause of his idea of God. Therefore, since the idea of the supremely perfect being is caused by God himself, necessarily God exists.

Hume has previously argued, however, that logical necessity is not relevant to the way reasoning about experience works. Moreover, a necessary connection between objects and events is not found in experience. "We are never able," Hume insists, "to discover any power or necessary connection ... which binds the effect to the cause, and renders the one an infallible consequence of the other."

There is no external impression of a necessary connection between events. There is also nothing of the sort internally. In other words, even the belief in the power to move one's own limbs is not derived from any internal impression. The idea or belief in such a power, Hume claims, "arises from reflecting on the operations of our own mind." It does not reveal "the secret union of soul and body, and the nature of both these substances; by which the one is able to operate ... upon the other."

Moreover, this presumed control is not complete. There are ways in which the body works that are not under the will's control. There has not, however, been any explanation of why some bodily events are and some are not under the will's control. Only the fact of experiencing the feeling of control has led to the idea that the will (or soul) causes some movements. The subsequent belief that one causes some of one's own movements is acquired in the same way as any other causal belief: "experience only teaches us, how one event constantly follows another; without instructing us in the secret connection" between them.

Causation and necessary connection are beliefs felt so strongly that any inferential move the mind made during the initial process of forming them is no longer considered. So, when one sees water, one immediately thinks about its buoyancy. When one sees fire, one immediately expects to feel heat, and so forth.

So, the idea of a necessary connection between events is not derived from the events themselves, since there is no impression of such a connection. In addition, repeated experience does not produce the idea, since each experience is complete. There is nothing added in multiple experiences that is not already in a single one. Consequently, since there is no impression of a necessary connection between events in a single sequence, there is no reason to think one develops over a series of similar sequences. Finally, the idea is not derived from reflection, that is, "copied from any internal impression." Just because one feels a "power" in willing some act, it does not follow that one feels the connection between the two, that is, between the will and its effects.

There are two parts to the idea of causation. One involves observation, the other, the mind. The two together explain why it is that some constantly conjoined events are not considered causally related. For example, one does not assert a causal relation between the sun rising and planes taking off at dawn. Lots of planes do take off at dawn, but this does not suggest to the mind that dawn causes them to take off—or vice versa. There are, of course, instances of false-cause reasoning. One can erroneously argue, for example, that police presence in a high-crime area is the cause of the crime.

Hume's point in offering two definitions seems to be that the legitimacy of causal claims is satisfied not only by the regularity of events, but also by conditions for belief. More specifically, the "customary transition" the mind makes from observed cause to anticipated effect, or from an observed effect to its cause, takes hold after repeated experiences.

The idea of necessary connection is a component of the idea of causation. Causation, Hume argues, is derived psychologically from the constant conjunction of events. So it is also with the idea of necessary connection: the mind becomes accustomed to the events occurring in a certain way.

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