In An Enquiry Concerning Human Understanding, David Hume applies his empiricist philosophy to a fundamental concept in metaphysics and science: causation. He famously argues that our idea of cause and effect, and our expectation that the same causes will produce the same effects, cannot be rationally justified – a point that leads to the problem of induction.
We take it for granted that one thing causes another: that dropping a glass causes it to smash, that leaving an ice cream out in the hot sun causes it to melt. And we take it for granted that these relationships will continue to hold into the future. But Hume questions whether we really can know such basic claims.
Do we experience causation?
Hume is a key figure in empiricism – the view that all knowledge and concepts ultimately come from experience.
So, if every concept comes from experience, then our concept of causation must come from experience too. But Hume asks: what sense perception, impression, or experience gives us this idea?
He considers one billiard ball hitting another as an example:
- We see the first ball (A) moving toward the second (B).
- We see them collide.
- We see the second ball (B) begin to move.
We take it for granted that A caused B to move but, Hume says, we don’t actually experience this causation. All we really see is one event happen (A moves and touches B) and then another event happen after (B moves). We see B follow A but we don’t see A cause B.
“It appears, then, that this idea of a necessary connexion among events arises from a number of similar instances which occur of the constant conjunction of these events; nor can that idea ever be suggested by any one of these instances, surveyed in all possible lights and positions. But there is nothing in a number of instances, different from every single instance, which is supposed to be exactly similar; except only, that after a repetition of similar instances, the mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist. This connexion, therefore, which we feel in the mind, this customary transition of the imagination from one object to its usual attendant, is the sentiment or impression from which we form the idea of power or necessary connexion. Nothing farther is in the case. Contemplate the subject on all sides; you will never find any other origin of that idea. This is the sole difference between one instance, from which we can never receive the idea of connexion, and a number of similar instances, by which it is suggested. The first time a man saw the communication of motion by impulse, as by the shock of two billiard balls, he could not pronounce that the one event was connected: but only that it was conjoined with the other.“
– Hume, An Enquiry Concerning Human Understanding, Section 7 Part 2 (emphasis added)
At no point do we actually perceive a “necessary connection” between the two billiard balls – some special power or force that makes the second ball move. At no point do we perceive causation. Instead, all we really observe are just separate events following one another in sequence.
So, says Hume, our senses never show us any necessary connection between cause and effect, only constant conjunction: that one type of event regularly follows another.
Causation is just in the mind
Hume argues that what we call causation – a necessary connection between events – is really just a mental habit formed through experience. We see one event follow another and, when we see a bunch of similar such instances, our mind begins to form an association between them.
After repeatedly observing one type of event (a ball being struck) followed by another (the second ball moving), we develop the expectation that the second will always follow the first:
“But when one particular species of event has always, in all instances, been conjoined with another, we make no longer any scruple of foretelling one upon the appearance of the other, and of employing that reasoning, which can alone assure us of any matter of fact or existence. We then call the one object, Cause; the other, Effect. We suppose that there is some connexion between them; some power in the one, by which it infallibly produces the other, and operates with the greatest certainty and strongest necessity. […]
After he has observed several instances of this nature, he then pronounces them to be connected. What alteration has happened to give rise to this new idea of connexion? Nothing but that he now feels these events to be connected in his imagination, and can readily foretell the existence of one from the appearance of the other. When we say, therefore, that one object is connected with another, we mean only that they have acquired a connexion in our thought, and give rise to this inference, by which they become proofs of each other’s existence: A conclusion which is somewhat extraordinary, but which seems founded on sufficient evidence.”
– Hume, An Enquiry Concerning Human Understanding, Section 7 Part 2 (emphasis added)
This habitual expectation is what gives rise to our idea of causation – but not any direct perception or experience of a causal power in the objects themselves. As Hume says, causation is a product of the imagination, formed through custom or habit, rather than something we can ever observe or know by reason.
In short, causation is a product of the mind, not something we perceive in the world. It’s a projection of our own expectations onto the patterns of experience.
The problem of induction
Hume’s analysis of causation leads to the problem of induction – the problem of how we can know that the patterns we’ve observed in the past will continue in the future, or that as-yet unobserved cases will resemble the ones we have observed.
For example, we believe that the sun will rise tomorrow because it rose yesterday, and the day before that, and the day before that, stretching back millions of years. But this belief relies on the assumption that nature is uniform – that the patterns and relations we’ve observed so far will continue to hold. But Hume asks: how do we justify this assumption that the future will resemble the past?
Firstly, our belief that the sun will rise tomorrow is not based on logical deduction. There’s no contradiction in imagining that the sun might not rise tomorrow. It’s logically possible that the Earth suddenly stops spinning, or that the laws of nature change overnight. So our belief in the uniformity of nature – that unobserved cases will resemble observed ones – cannot be established by reason alone.
Secondly, this belief can’t be justified by experience either. Any attempt to justify the uniformity of nature through past experience would itself rely on the very principle it’s trying to prove! We would be arguing that because the past has resembled the future in the past, it will continue to do so, which is circular reasoning.
So, says Hume, there is no rational justification for induction. All inductive reasoning – reasoning that goes from past experience to future expectation – rests on custom or habit, not on logic or reason.
“As to past Experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist. The bread, which I formerly eat, nourished me; that is, a body of such sensible qualities was, at that time, endued with such secret powers: but does it follow, that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers? The consequence seems nowise necessary. At least, it must be acknowledged that there is here a consequence drawn by the mind; that there is a certain step taken; a process of thought, and an inference, which wants to be explained. These two propositions are far from being the same, I have found that such an object has always been attended with such an effect, and I foresee, that other objects, which are, in appearance, similar, will be attended with similar effects. I shall allow, if you please, that the one proposition may justly be inferred from the other: I know, in fact, that it always is inferred. But if you insist that the inference is made by a chain of reasoning, I desire you to produce that reasoning.”
– Hume, An Enquiry Concerning Human Understanding, Section 4 Part 2 (emphasis added)
When we see the sun rise every morning, our mind becomes accustomed to that pattern and develops the expectation that it will happen again. When eating bread has satisfied our hunger all those times before, we expect it will do so next time. This expectation feels like knowledge but, like causation, Hume says it’s only psychological – a product of mental habit, not rational insight or direct experience.
A sceptical conclusion
So although Hume’s empiricism says knowledge originates in experience, his analysis of causation and induction highlights the limits of empiricism. We trust experience to guide our predictions, yet experience alone offers no justification for that trust. Hume’s examination of causation and the problem of induction together suggest limits to human understanding. We never perceive causal necessity in the world, only regularity. And what’s more, we cannot rationally justify the belief that this regularity will continue.
This leads to a deeply sceptical conclusion: not only can we never directly observe the necessary connection between cause and effect, but we also cannot logically guarantee that past patterns will repeat in the future. And this challenges the very foundations of our reasoning about the world and of science.
Despite this scepticism, we must act as if causal connections and inductive inferences are reliable. Induction may not be rationally grounded but Hume says it is psychologically unavoidable:
“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.
Without the influence of custom, we should be entirely ignorant of every matter of fact beyond what is immediately present to the memory and senses. We should never know how to adjust means to ends, or to employ our natural powers in the production of any effect. There would be an end at once of all action, as well as of the chief part of speculation. […]
This belief is the necessary result of placing the mind in such circumstances. It is an operation of the soul, when we are so situated, as unavoidable as to feel the passion of love, when we receive benefits; or hatred, when we meet with injuries. All these operations are a species of natural instincts, which no reasoning or process of the thought and understanding is able either to produce or to prevent.”
– Hume, An Enquiry Concerning Human Understanding, Section 5 Part 1
Summary
- Hume says all our ideas come from experience.
- But Hume says we never actually experience the idea of causation:
- We experience B follow A (e.g. one billiard ball move after being struck by another).
- But we don’t experience A causing B.
- However, when we see B follow A enough times, our mind projects this causal connection on to experience. We expect this connection to hold in the future.
- But Hume also says experience can’t justify the expectation that these patterns will continue into the future (the problem of induction):
- Reason doesn’t justify induction: Just because the sun has risen every day for millions of years, there is no logical contradiction in the sun not rising tomorrow.
- Experience doesn’t justify induction either: If we say the sun will rise tomorrow because it has always risen, then we’re assuming the very thing we’re trying to prove! We’re assuming nature is consistent in order to prove nature is consistent.
- So even though Hume’s empiricism explains the source of our ideas, it also reveals their limits: No amount of experience can give us certainty. Our expectations are grounded in habit and custom, not in logical proof, which means that all knowledge derived from experience is at best probable rather than certain.