An Enquiry Concerning Human Understanding

Examines two approaches to moral philosophy: the practical approach focused on virtue and the abstract approach investigating human nature's foundations.

All ideas originate as copies of impressions from sensation or reflection; complex ideas derive from simple ones.

Ideas associate through three principles: resemblance, contiguity in time or place, and cause or effect.

Reasoning about matter of fact rests on cause and effect, which we know only through experience, not reason.

Custom and habit, not reason, determine our inferences from experience and produce belief in causal connections.

Probability arises from the proportion of contrary chances and observed uniformity in causes producing effects.

Necessary connection is not observable in nature but arises from custom linking frequently conjoined objects.

Human actions exhibit the same necessity as physical events; both liberty and necessity are essential to morality.

Animals reason from experience through custom and instinct, similar to human experimental reasoning.

No human testimony can establish miracles, since uniform experience against them provides perfect counter-proof.

Arguments from design cannot establish divine providence or justice beyond what nature visibly displays.

Mitigated skepticism usefully limits inquiry to common life and observable facts, rejecting metaphysical speculation.