In Section VII of An Enquiry Concerning Human Understanding, 18th century philosopher David Hume continues his revolutionary natural philosophy of the mind by examining the limits of human understanding with respect to ideas of causation:
When we look about us towards external objects, and consider the operations of causes, we are never able, in a single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. (EHU 7.6)
Departing radically from the rationalist epistemology of his predecessors, Hume argues that underlying notions of causation as it would exist in nature, such as force, energy, and power, are beyond the reach of human understanding if not completely meaningless. Hume shows that our ideas of causation are inadequate by examining and then discrediting the notions of causation with which we are most familiar: the power we ascribe the will over movements of the body, and the power we believe the mind has to effect ideas within itself. Although Hume argues that human understanding excludes the underlying notions of causation as it would exist in nature, he accounts for our notion of cause and effect using the concept of impressions of constant conjunction taught by experience, thus revealing an intuitive conception of causation compatible with his naturalist account of the human mind.
The Will's Power Over the Body
In support of his claim that the mind has an inadequate understanding of causation as it would exist in nature, Hume evaluates a causal notion with which all able-bodied humans are familiar – the will's influence over movement of the body – and shows how the notion that the will possesses an inherent power to effect such movements is obscure and confused, and that we cannot establish a necessary connection between acts of will and bodily movements. Concerning our notions of causation between bodies external to us, Hume remarks "as we are totally ignorant of [this] power…we are no less ignorant of that power, on which depends the operation of mind on body" (EHU 7.21). Hume illustrates the inadequacy of our notion of the will's power over the activities of the body, stating that if we really could perceive such a power, we would understand "the secret union of soul and body, and the nature of both these substances" (EHU 7.11), and we know that we have no such knowledge. Also, the notion of such a power implies a necessary connection between the object we perceive to be the cause and the object we perceive to be the effect. Hume argues that, as with all matters of fact, no logical contradiction is implied when one conceives an effect other than the intended bodily movement resulting from an act of will; therefore, no necessary connection exists.
To further discredit the notion of necessary connection implied by our ideas of causation, Hume considers the complex chain of events that link an act of volition with a movement of body:
The mind wills a certain event: Immediately another event, unknown to ourselves, and totally different from the one intended, is produced: This event produces another, equally unknown: Till at last, through a long succession, the desired event is produced…that their motion follows the command of the will is a matter of common experience…but the power or energy by which this is effected, like that in other natural events, is unknown and inconceivable. (EHU 7.14-15)
Although we feel that an act of our will causes movement in our body, we find that the result does not follow from the cause directly; there exists an intermediate, complex chain of causes and effects, most of which are not fully understood. To show that we lack a meaningful notion of causation as it would exist in nature, Hume demonstrates that one of our most fundamental notions of causation is confused, and we conclude from this that our more foreign notion of causation as it would exist in nature is inadequate.
The Mind's Power Over Ideas
Furthering his argument that we lack an adequate notion of causation as it would exist in nature, Hume shows what many would consider to be the most familiar notion of causation to the human understanding, the mind's power to effect ideas within itself, to be obscure and confused, so that even the most intimate idea of causation "gives us no real idea of force or energy" (EHU 7.16). After showing our concept of the power with which the will causes movement in the body to be obscure and confused, Hume begins to discredit our most fundamental notion of causation: "a power or energy in our own minds, when, by an act or command of our will, we raise up a new idea" (EHU 7.11). Hume believes that we cannot possibly understand this power, because in order to comprehend the means by which an idea is produced, we would also have to understand true creation, or "production of something out of nothing" (EHU 7.17), a capacity Hume reserves for an Infinite Intellect alone.
Instead of understanding the power the mind has over its own affects, we only have strong feeling of necessary connection between commands of the will and the arrival of ideas in the mind caused by impressions of the conjunction of those events; we have experienced that every time our will commands an idea into mind, an idea soon follows. Although we form this notion of necessary connection in our minds, Hume restates that this power as it would exist in nature is "entirely beyond our comprehension" (EHU 7.17). We are only aware of this connection by what experience has taught us, because when we attempt to link an act of the will with the arrival of an idea in the mind, as attempted previously with an act of will and a movement in the body, we are unable to perceive the connection, and we realize that our notion of the mind's power over its own states is obscure, and, similarly, that no necessary connection exists. After Hume demonstrates that even our most intimate notion of causation is confused, we realize that our notion of causation as it would exist in nature is inadequate.
Constant Conjunction as the Foundation
Although Hume demonstrates that human understanding excludes the idea of causation as it would exist in nature, he provides a phenomenological foundation for our ideas of cause and effect by attributing them to notions of constant conjunction. Throughout Section VII, Hume exhausts his claim that "even in the most familiar events, the energy of the cause is as unintelligible as in the most unusual" (EHU 7.21) by showing that even the most intuitive notions of causation – those between our will, the movements of our body, and the states of our mind – are merely confused ideas. If these fundamental notions of causation are so unclear, how do we account for the notions of cause and effect upon which we form nearly every decision we make, and in accordance with which we conform every facet of our lives?
Hume explains that we are taught notions of cause and effect by experience, where the frequent conjunction of certain impressions enables us to establish causal relations in our minds without actually understanding the secret powers with which the objects of our impressions actuate one another. Hume then offers two phenomenological definitions of cause in the context of human understanding:
We may define a cause to be an object, followed by another, and where all the objects, similar to the first, are followed by objects similar to the second… [or] whose appearance always conveys the thought to that other. (EHU 7.29)
Essentially, Hume reveals the inadequacy of our idea of causation as it would exist in nature by explaining that we often form such lively feelings of connections between objects that we believe these connections to be properties of the objects themselves, independent of our minds and occurring naturally. In this way, Hume provides an account for our ideas of cause and effect compatible with his natural philosophy of the mind while upholding his claim that our notion of causation as it would exist in nature is inadequate.
Conclusion
In his exploration of the limits of human understanding, Hume offers an appealing argument that causation as it would exist in nature is unknowable. Hume shows that instead of possessing an adequate idea of causation, we are taught by experience to form ideas of cause and effect from the constant conjunction of impressions. Hume concludes that we may understand causation to consist in either the constant conjunction of objects in experience, or the constant conjunction of ideas in our understanding, but concerning the powers that underlie these phenomenological conceptions of causation, "all we know is our profound ignorance" (EHU 7.25).
