An Enquiry Concerning Human Understanding

1. All our thoughts are copies of our experiences.

Or, to express myself in philosophical language, all our ideas or more feeble perceptions are copies of our impressions or more lively ones.

Ideas come from impressions. Hume argues that all the contents of our minds, which he calls "perceptions," can be divided into two categories: impressions and ideas. Impressions are our vivid, immediate sensations and feelings (like seeing red or feeling pain). Ideas are the fainter copies of these impressions that we use when we think or imagine (like remembering seeing red or thinking about pain).

No innate ideas. This principle, known as the Copy Principle, means that we cannot have any idea that we haven't first experienced as an impression. For example, we can imagine a golden mountain by combining the idea of gold (from seeing gold) and the idea of a mountain (from seeing mountains), but we cannot conceive of a completely new sensation or feeling we've never experienced.

• Impressions are lively and strong (sensations, feelings).
• Ideas are faint and weak (thoughts, memories, imaginings).
• Every idea is derived from a corresponding impression.

A tool for clarity. Hume proposes using this principle as a "new microscope" to clarify obscure philosophical terms. If a term is used without a clear meaning, we should ask: "From what impression is that supposed idea derived?" If no original impression can be found, the term is likely meaningless or confused.

2. Knowledge is either certain (ideas) or probable (facts).

All the objects of human reason or enquiry may naturally be divided into two kinds, to wit, Relations of Ideas, and Matters of Fact.

Two kinds of truth. Hume divides all possible objects of human knowledge into two distinct categories. The first, "Relations of Ideas," concerns truths that are logically necessary and discoverable purely by thinking, without needing to look at the world. Examples include mathematics and logic.

• Examples: 2 + 2 = 4, All bachelors are unmarried, A triangle has three sides.
• These truths are intuitively or demonstratively certain.
• Their negation implies a contradiction.

Facts require experience. The second category, "Matters of Fact," concerns truths about what exists or happens in the world. These cannot be known purely by thought; they require experience or observation. The opposite of any matter of fact is always conceivable and implies no contradiction.

• Examples: The sun will rise tomorrow, Fire causes heat, Caesar crossed the Rubicon.
• These truths are known only through experience.
• Their negation is always possible and conceivable.

The limit of reason. This distinction, often called Hume's Fork, is fundamental to his philosophy. It establishes that abstract reasoning alone can only give us certainty about the relationships between our ideas, but any knowledge about the real world must ultimately be based on experience.

3. Reason cannot prove the future will resemble the past.

If we be, therefore, engaged by arguments to put trust in past experience, and make it the standard of our future judgment, these arguments must be probable only, or such as regard matter of fact and real existence... But that there is no argument of this kind, must appear...

The problem of induction. Hume poses a profound challenge to how we justify our beliefs about the unobserved world. All our knowledge of facts beyond our immediate senses and memory relies on inferring effects from causes, or vice versa. This inference, in turn, relies on past experience – we expect similar effects from similar causes.

No logical justification. But how do we justify the crucial assumption that the future will be like the past? Hume argues this cannot be proven by demonstrative reasoning (like logic or math) because it's perfectly conceivable that the course of nature could change. Nor can it be proven by probable reasoning (based on experience) because any such argument would already have to assume that past experience is a reliable guide to the future, leading to circular reasoning.

• Demonstrative proof: Impossible (future could be different).
• Probable proof: Circular (assumes what it tries to prove).

A step without reason. Therefore, Hume concludes, the step we take in inferring the future from the past is not based on any process of the understanding or chain of argument. It is a fundamental assumption that reason cannot justify.

4. Our belief in cause and effect is driven by custom, not logic.

Custom, then, is the great guide of human life.

The source of inference. Since reason cannot justify our inferences about the future, what does? Hume identifies "Custom or Habit" as the principle that determines us to expect similar effects from similar causes. When we repeatedly observe two events constantly conjoined (like fire and heat), our minds are led by habit to expect the second upon the appearance of the first.

An instinctual process. This expectation is not a conclusion reached through deliberation or argument; it's an automatic, instinctual process. Just as nature teaches us to use our limbs without understanding anatomy, it implants in us an instinct to infer effects from causes based on past conjunctions.

• Custom is a principle of human nature.
• It operates automatically, without reasoning.
• It is essential for survival and action.

Belief as a feeling. This customary transition of thought leads not just to a conception of the expected effect, but to a belief in its existence. Hume describes belief as a peculiar sentiment or feeling – a more vivid, lively, or forceful conception of an idea compared to mere imagination. This feeling is produced by the mind's habitual determination to pass from the cause to its usual effect.

5. The idea of 'necessary connection' is an internal feeling.

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.

Seeking the impression. Following his Copy Principle (Section II), Hume seeks the original impression from which our idea of "power, force, energy, or necessary connexion" is derived. He examines instances of causation in the external world (like billiard balls colliding) and in our internal world (like willing our body to move or our mind to think).

No external impression. In external objects, we only observe one event following another – constant conjunction – but never any "power or necessary connexion" binding them together. We cannot foresee the effect from the cause without experience, proving the connection is not discoverable in the objects themselves.

• We see contiguity and succession.
• We see constant conjunction over many instances.
• We do not see or feel a necessary link in any single instance.

No internal impression. Similarly, we are not conscious of any power in our will to move our body or control our thoughts. We only know by experience that these effects follow our volitions. The means by which mind and body interact, or mind controls itself, are mysterious and unknown to us.

The internal feeling. The only remaining source for the idea of necessary connection is the internal feeling that arises after observing many instances of constant conjunction. This feeling is the mind's habitual determination or customary transition from the appearance of one object to the expectation and belief of its usual attendant. This internal sentiment is the impression from which the idea of necessary connection is copied.

6. Human actions are as predictable as physical events.

It seems almost impossible, therefore, to engage, either in science or action of any kind, without acknowledging the doctrine of necessity, and this inference from motives to voluntary actions; from characters to conduct.

Uniformity in human behavior. Hume argues that human actions exhibit a regularity and uniformity comparable to the operations of matter. While there are variations due to differences in character, circumstances, and motives, these variations themselves often follow predictable patterns.

• History shows consistent human nature across ages and nations.
• Experience allows us to infer motives from actions and vice versa.
• We rely on this predictability in daily life (e.g., expecting a merchant to sell goods).

Necessity in human affairs. Applying his definition of necessity (constant conjunction and consequent inference) to human actions, Hume concludes that they are subject to the same necessity as physical events. We observe constant conjunctions between motives, circumstances, and actions, and we constantly make inferences based on these conjunctions.

• Constant conjunction: Particular actions are regularly united with particular motives.
• Inference of the mind: We infer actions from motives and characters.

Philosophers and the vulgar agree. Both ordinary people and philosophers implicitly acknowledge this necessity in their practical lives and theoretical pursuits (history, politics, morals, criticism). The apparent reluctance to admit necessity in words stems from a misunderstanding of what necessity means, often mistakenly associating it with a mysterious power perceived in matter, which Hume denies exists.

7. Freedom means acting according to your will, which is compatible with necessity.

By liberty, then, we can only mean a power of acting or not acting, according to the determinations of the will; that is, if we chuse to remain at rest, we may; if we chuse to move, we also may.

Defining liberty. Hume proposes a definition of liberty that he believes is consistent with both plain fact and itself, and which all mankind can agree upon. He defines liberty, when applied to voluntary actions, as the power to act or not act according to the choices or determinations of one's will.

• Liberty is the ability to do what you will.
• It is opposed to constraint (being physically forced or prevented).
• It is not opposed to necessity (causal determination).

Compatibility with necessity. This definition of liberty is perfectly compatible with the doctrine of necessity (that actions are causally determined by motives and circumstances). If our actions follow from our will, we are free in this sense, even if our will itself is causally determined.

• Our actions follow our will (this is liberty).
• Our will follows our motives/circumstances (this is necessity).
• These two facts do not contradict each other.

Essential for morality. Furthermore, Hume argues that this necessity is essential for morality. Blame and punishment are only appropriate if actions proceed from durable causes in the agent's character and disposition. If actions were random or uncaused (the opposite of necessity), they could not be attributed to the person and would not reflect their moral character.

8. We must weigh testimony against the probability of the event, especially for miracles.

A wise man, therefore, proportions his belief to the evidence.

Evidence from testimony. Our belief in testimony is based on our experience of the veracity of witnesses and the usual conformity of facts to reports. Like all other inferences from experience, it varies in strength depending on the uniformity of past observations. We consider factors like the witness's character, number, manner of delivery, and interest.

Extraordinary claims require extraordinary evidence. When testimony reports an extraordinary or marvellous event, the evidence from testimony is weighed against the evidence from our experience of how often such events occur. The more unusual the event, the greater the evidence required from the testimony to overcome the presumption against it.

Miracles violate natural laws. A miracle is defined as a violation of a law of nature. Since laws of nature are established by firm and unalterable experience (amounting to a "proof"), there is a full proof from the nature of the fact against the occurrence of any miracle.

• Proof against a miracle: Based on uniform experience of natural laws.
• Proof for a miracle: Based on testimony.
• These proofs are opposed and must be balanced.

Testimony for miracles is insufficient. Hume's famous conclusion is that no testimony is sufficient to establish a miracle unless its falsehood would be more miraculous than the event it reports. He argues that, in practice, testimony for religious miracles is never strong enough, citing factors like:

• Lack of credible witnesses.
• Human propensity for wonder and religious enthusiasm.
• Miracles occurring mostly in ignorant ages/nations.
• Miracles of different religions cancelling each other out.

9. We cannot infer God's attributes or future actions beyond what is seen in the world.

Let your gods, therefore, O philosophers, be suited to the present appearances of nature: And presume not to alter these appearances by arbitrary suppositions, in order to suit them to the attributes, which you so fondly ascribe to your deities.

Design argument limitations. Hume, through the voice of his character Epicurus, critiques the attempt to infer God's nature and future actions from the observed order of the universe (the Design Argument). He argues that when we infer a cause from an effect, we must proportion the cause exactly to the effect.

• The cause inferred can only have qualities precisely sufficient to produce the observed effect.
• We cannot ascribe further qualities or degrees of qualities beyond what is manifested in the effect.

No inference from cause to new effects. Since our knowledge of the divine cause is derived solely from the observed effects (the universe), we cannot then turn around and use this inferred cause to predict or infer new effects (like a future state of rewards and punishments) that are not already evident in the world.

• We infer cause from effect (e.g., intelligence from order).
• We cannot infer new effects from the inferred cause (e.g., a perfect future state from inferred perfect intelligence).

Against projecting human ideals. Philosophers often project their own ideals of perfection onto the divine being and then assume the world must conform to these ideals, despite the presence of evil and disorder. Hume argues this is an arbitrary supposition, not based on the evidence of nature itself. The religious hypothesis can only account for visible phenomena; it cannot justify inferring any new facts or altering our understanding of the world beyond what experience shows.

10. Excessive scepticism is impractical; a modest approach is the guide of life.

The great subverter of Pyrrhonism or the excessive principles of scepticism, is action, and employment, and the occupations of common life.

The limits of doubt. Hume distinguishes between different forms of scepticism. Excessive or Pyrrhonian scepticism aims at universal doubt, questioning the reliability of our senses and reason entirely. While the arguments for this extreme scepticism may be theoretically irrefutable, Hume argues they are utterly impractical.

• Cartesian doubt (antecedent scepticism): Impossible to achieve and incurable.
• Pyrrhonian doubt (consequent scepticism): Theoretically strong, but vanishes in practice.

Nature overcomes reason. Human nature is too strong to allow for total scepticism. Our instincts and necessities compel us to act, reason, and believe, regardless of philosophical doubts. A Pyrrhonian sceptic cannot function in the world and would perish if they consistently applied their principles.

Mitigated scepticism. However, a more moderate or "mitigated" scepticism can be both durable and useful. This "Academical" philosophy arises from reflecting on the limitations of human understanding, leading to modesty and caution in our judgments.

• Inspires modesty and diffidence.
• Corrects dogmatism and prejudice.
• Leads to a limitation of inquiry.

Confining inquiry. Mitigated scepticism encourages us to confine our inquiries to subjects best adapted to our limited faculties – those within the scope of common life and experience. It teaches us that while philosophical reflection can methodize and correct common reasoning, it cannot take us beyond the bounds of experience into obscure metaphysical or theological subjects.

11. Worthwhile inquiry is limited to abstract reasoning about quantity or experimental reasoning about facts.

When we run over libraries, persuaded of these principles, what havoc must we make?

The scope of human understanding. Based on his analysis of the different types of reasoning, Hume proposes a strict limitation on the legitimate subjects of human inquiry. He argues that there are only two types of reasoning that can yield knowledge:

• Abstract reasoning concerning quantity or number (mathematics).
• Experimental reasoning concerning matter of fact and existence (empirical science).

Mathematics and empirical science. Abstract reasoning is limited to mathematics because only ideas of quantity and number have the precise and intricate relations necessary for demonstration. All other ideas are distinct, and reasoning about them amounts to little more than defining terms. Experimental reasoning, based on cause and effect and custom, is the only way to learn about the real world.

• Mathematics: Certain, but only about relations of ideas.
• Empirical Science: Probable, about matters of fact.

Condemnation of metaphysics and divinity. Any purported knowledge that falls outside these two categories is suspect. This includes traditional metaphysics and much of theology, which attempt to establish matters of fact (like God's existence or the soul's immortality) through abstract reasoning or to go beyond experience in their claims about reality.

• Metaphysics: Often deals with subjects inaccessible to understanding.
• Divinity: Often relies on abstract reasoning or claims beyond experience.

Hume's Bonfire. This leads to Hume's famous concluding challenge: if a book contains neither abstract reasoning about quantity nor experimental reasoning about fact, it contains only "sophistry and illusion" and should be committed to the flames. This is a call to reject speculative philosophy that pretends to knowledge beyond the reach of human experience and mathematical demonstration.

Last updated: May 11, 2025