Unit 3: Overview
What if reason can’t justify the things we most rely on it to know?
This page is organized into five sections. Click any heading to open it — you can have multiple sections open at once.
What this unit is about
The sun has risen every morning of your life. Every morning before your life. Every morning anyone has ever recorded. So it will rise tomorrow. Right?
Probably. But David Hume wants to know: what actually justifies that conclusion? You have seen the past. You have not seen the future. The fact that something has always happened before is not, strictly speaking, a logical guarantee that it will happen again. The future is not the past. So on what basis do we feel so confident?
This is not a trick question. It is the heart of Hume’s philosophy — and the more you think about it, the stranger it gets. Hume’s great contribution is to take this strangeness seriously and follow it to its conclusions, however unsettling those conclusions turn out to be.
The shape of the argument
Hume begins from a position called empiricism: the view that all genuine knowledge is ultimately grounded in experience. We have two kinds of mental content, he says — impressions, which are the direct deliverances of the senses and emotions, and ideas, which are the mind’s fainter copies of those impressions. Anything that cannot be traced back to an original impression is philosophically suspect.
That standard does serious damage to things philosophers have long taken for granted. Take causation. When you see one billiard ball strike another and the second ball move, you feel certain that the first ball caused the second to move. But what exactly did you see? You saw one event followed by another. You did not see any force, any power, any necessary connection between them. You saw succession. And from repeated successions, your mind formed a habit of expectation — nothing more.
This is Hume’s skepticism: not that we can’t know anything (he is quite happy to rely on experience as a practical guide), but that the foundations of our most confident beliefs are much shakier than we thought. Reason, which Descartes had vindicated as the instrument of certain knowledge, turns out to have very limited reach. The things we know best — that the sun will rise, that fire will burn, that the future will resemble the past — are not products of reason at all. They are habits of mind.
Why it matters
Hume’s challenge to causation and induction goes to the heart of how science works. Scientists observe patterns and infer laws. Hume asks: can that inference ever be fully justified? His answer — that it cannot be justified by reason alone, only by habit and custom — is one of the most consequential arguments in the history of philosophy. It famously woke Immanuel Kant from what he called his “dogmatic slumber” and set the agenda for philosophy in the century that followed.
← Looking back | Looking ahead →
In Unit 2, Descartes set out to secure knowledge on a foundation of rational certainty. He doubted everything he could doubt, found the one thing he couldn’t — the fact of his own thinking — and rebuilt from there, with God as guarantor of reason’s reliability. Hume’s empiricism is the counter-attack: if all knowledge must ultimately trace back to experience, then reason alone — without sensory grounding — tells us almost nothing. The certainty Descartes thought he had secured turns out to rest on habit.
In Unit 4, Hegel will respond to the problem Hume opened up. Where Hume concludes that reason cannot reach beyond experience, Hegel will argue that reason does not merely describe reality — it constitutes it. If Hume empties reason out, Hegel tries to fill it back in on an entirely different basis.
Before you read: watch this first
The Enquiry is more reader-friendly than the Meditations, but Hume’s core arguments can feel slippery on a first pass because he is asking you to question things that seem obvious. This video introduces empiricism, the problem of induction, and Hume’s analysis of causation before you encounter them in the text.
The School of Life: “David Hume” (~11 min). Captions available on YouTube.
Before you continue
Think of something you are completely confident will happen tomorrow — the sun rising, your phone turning on, water flowing from the tap. Now ask: why are you confident? What is the actual basis of that confidence? Hold that question in mind as you read. Hume’s answer will probably surprise you.
What to notice in the video
— The distinction between impressions and ideas, and why it matters
— How Hume’s “fork” divides all knowledge into two types — and what falls outside both
— Why Hume thinks our belief in causation cannot be justified by reason alone
— What “custom and habit” means as an alternative explanation
Unit reading
This unit has one primary text, assigned in five sections. The Enquiry Concerning Human Understanding is Hume’s own condensed and clarified version of his earlier, longer Treatise of Human Nature. It is deliberately written for a general audience and is, by the standards of this course, quite readable. Plan to read each section carefully — they are short but dense.
Primary text
David Hume, Enquiry Concerning Human Understanding
Trans. Jonathan Bennett, Early Modern Texts, 2017 (modernized English)
Assigned: Sections 1–5 (approximately 35 pages)
Available free online: earlymoderntexts.com/assets/pdfs/hume1748.pdf
Reading guide
Section 1 — “Different Species of Philosophy.” Hume distinguishes the kind of philosophy he is doing — rigorous, empirically grounded — from what he sees as the vague, speculative variety. Short but important for orientation.
Section 2 — “Origin of Ideas.” Introduces the impressions/ideas distinction and the Copy Principle. This is the empiricist foundation everything else rests on.
Section 3 — “Association of Ideas.” Three principles explain how the mind connects ideas: resemblance, contiguity, and cause and effect. Short; read it carefully.
Section 4 — “Sceptical Doubts.” The most important section. Hume introduces his famous fork dividing all knowledge into “relations of ideas” and “matters of fact,” then argues that our belief in cause and effect cannot be justified a priori. Read it twice if you can.
Section 5 — “Sceptical Solution.” Hume’s answer to his own challenge: if reason can’t justify our causal inferences, what does? Custom and habit. This section also contains his account of belief.
Reading tip
Hume sometimes writes in a voice of confident, almost breezy skepticism — as though the unsettling conclusions he is reaching are perfectly natural. Don’t let that tone make it feel easy. Pause when something seems obvious and ask: is it actually obvious, or does Hume have a point?
Handouts for this unit
These PDFs accompany the reading. Download them before you begin — they are useful to have alongside the text.
Accessibility
If you use a screen reader or require an alternative format for any of these documents, please contact your instructor. All course materials can be made available in an accessible format upon request.
What you need to complete this unit
Work through each item in order.
- Watch the introductory video
- Read the lesson pages in this unit
- Read the assigned text and review the handouts
- Complete the reflection prompt
- Take the unit quiz
In This Unit
- Hume's Life and Context
- Impressions, Ideas, and Association
- Causation, Induction, and the Limits of Reason
- Reflection Prompt
- Quiz: David Hume