The Limits of Reason: Hume and the Problem of Cause and Effect

David Hume’s An Enquiry Concerning Human Understanding is one of the most quietly devastating books in philosophy, all the more so because Hume writes with such calm, good-humoured clarity. Where Locke had argued that knowledge comes from experience, Hume followed the empiricist principle to its ruthless conclusions and discovered just how little experience can actually justify. With genial relentlessness, he shows that some of our most basic assumptions, that the future will resemble the past, that causes produce effects, rest on no rational foundation at all. It is a book that famously woke Immanuel Kant from his “dogmatic slumber,” and reading it, I understood why.

Here is what Hume taught me about the problem of cause and effect, about the limits of reason, and about the role of habit in everything we believe.

Key takeaways

• We never actually observe causation, only one event followed by another.
• The belief that the future will resemble the past cannot be proved by reason.
• Custom and habit, not reason, are the great guides of human life.
• Reason is humbler than we think, and much of belief rests on instinct.
• Skepticism, held moderately, can make us wiser and less dogmatic.

The hidden gap in cause and effect

Hume’s most famous argument concerns causation, and it is deceptively simple. When one billiard ball strikes another and the second moves, we say the first caused the second. But what do we actually observe? Only one event followed by another: the first ball moves, then the second moves. We never see the “causation” itself, the necessary connection that supposedly binds them. We see succession, and we infer cause. Hume’s unsettling point is that this inference cannot be justified by reason. There is nothing in the first event that logically guarantees the second; we have simply seen them conjoined many times. This little observation, calmly made, knocks a hole in the foundations of science and everyday knowledge alike.

Why we cannot prove the future

The damage deepens with what philosophers now call the problem of induction. All our reasoning about the world assumes that the future will resemble the past, that bread will nourish tomorrow as it did today, that the sun will rise again. But how do we know this? Not by logic, for there is no contradiction in imagining a future that differs wildly from the past. And not by experience, for that would be circular: to argue that the future will resemble the past because it always has is already to assume the very thing in question. Hume concludes, astonishingly, that our most basic expectations about the world have no rational justification whatever. I found this genuinely vertiginous. The ground I assumed was solid turned out to rest on nothing reason can supply.

Habit, the great guide of life

And yet Hume is no paralysing skeptic. Having shown that reason cannot justify our basic beliefs, he asks why we hold them anyway, and answers: custom, or habit. We expect the future to resemble the past not because we have proved it but because our minds are built to form such expectations after repeated experience. Custom, he writes, is the great guide of human life. This is not a defeat but a relocation. Hume moves the foundation of our knowledge from reason, which cannot bear the weight, to human nature, to the instincts and habits that make belief automatic. We are creatures of habit far more than of pure logic, and this is what allows us to live and act at all. I find something deeply sane in this. Hume dismantles the pretensions of reason, then gently reassures us that nature has equipped us to get on regardless.

What I’m taking with me

Hume is the most disarming of radicals, demolishing the foundations of knowledge with a smile and then inviting us to dinner. What I am taking from the Enquiry is a lasting humility about the reach of reason, an awareness that even my most basic certainties rest on habit rather than proof, and a healthier, gentler skepticism that holds its beliefs more lightly. Hume does not want us to despair or to stop living; he wants us to stop being arrogant about how much we really know. That seems to me a permanently useful lesson, and a strangely cheerful one.

Frequently asked questions

What is Hume’s Enquiry Concerning Human Understanding about?

It examines the origins and limits of human knowledge, arguing that we cannot rationally justify our belief in cause and effect or that the future will resemble the past, and that custom and habit, not reason, guide our beliefs.

What is the problem of induction?

Hume’s point that our assumption that the future will resemble the past cannot be proved, since logic does not require it and appealing to past experience to justify it is circular.

What does Hume say guides human life, if not reason?

Custom, or habit. We form expectations and beliefs through repeated experience rather than rational proof, and these habits, built into human nature, allow us to act in the world.

References & further reading

1. Hume, David. An Enquiry Concerning Human Understanding. (1748; edited by L. A. Selby-Bigge and P. H. Nidditch, Oxford University Press.)
2. Stroud, Barry. Hume. Routledge, 1977.
3. Ayer, A. J. Hume. Oxford University Press, 1980.