Summary
Section I: Different species of philosophy
Hume contrasts practical, eloquent philosophy with abstract inquiry into human nature. He seeks a union of clarity and depth that improves common judgement while exposing superstition and meaningless speculation.
Section II: Origin of ideas
Perceptions divide into vivid impressions and fainter ideas. Ideas generally copy prior impressions; sensory deprivation removes the relevant concepts. The missing shade of blue is acknowledged as a rare exception.
The copy principle tests philosophical language: an unclear idea should be traced to the impression from which it derives.
Section III: Association of ideas
Thought is connected by resemblance, contiguity and cause and effect. These principles explain transitions in memory, conversation and narrative without making association perfectly mechanical.
Section IV: Sceptical doubts
Relations of ideas are demonstratively certain and independent of actual existence. Matters of fact are contingent, and reasoning beyond observation depends upon cause and effect.
Causes cannot be discovered a priori. Experience supplies repeated conjunction, but no demonstrative proof that the future must resemble the past. Any experiential proof of uniformity assumes the principle it tries to establish.
Section V: Sceptical solution
Custom produces expectation after repeated experience. Belief is a lively manner of conceiving an idea, connected through habit with a present impression.
Hume treats this mechanism as a wise provision of nature: life depends upon causal anticipation before reflective reasoning begins.
Section VI: Probability
Belief varies in strength according to frequencies, analogies and contrary experiences. Repeated outcomes distribute expectation, while the imagination transfers vivacity towards the more supported possibility.
Section VII: Necessary connexion
No impression of external force is discovered by inspecting objects or voluntary action. Repetition creates an internal determination to pass from cause to effect.
Cause can be defined through constant conjunction and through the inferential transition one event produces in the mind. Necessity consists in these observable and psychological relations.
Section VIII: Liberty and necessity
Human conduct displays regular connections with motives, character and circumstances. This is the same necessity recognised in physical causation.
Liberty is the ability to act according to one’s will when unconstrained. Properly defined necessity and liberty are compatible and jointly support moral responsibility.
Section IX: Reason of animals
Animals learn regularities, anticipate outcomes and alter behaviour through experience. Their conduct supports the view that causal inference arises from custom rather than demonstrative reasoning.
Section X: Miracles
A wise person proportions belief to evidence. A miracle conflicts with established laws of nature, so testimony for it faces the counter-evidence of uniform experience.
Witness reliability, cultural enthusiasm, love of wonder and conflicts among religions further weaken reported miracles. Testimony could in principle be weighed, but Hume finds no religious report sufficient.
Section XI: Providence and a future state
Through a dialogue, Hume examines arguments from the order of the world to divine attributes. Effects justify only proportionate causes; finite mixed experience cannot establish an infinite, perfect providence.
One cannot add unobserved divine qualities and then infer future rewards or purposes from them without exceeding the evidence.
Section XII: Academical scepticism
Antecedent Cartesian doubt cannot be escaped if genuinely universal, while excessive scepticism about reason and senses cannot govern life. Nature restores ordinary belief.
A mitigated scepticism limits confidence, checks dogmatism and directs inquiry towards subjects suited to human faculties. Demonstration applies to quantity; empirical reasoning governs matters of fact and existence.
The closing attack on unsupported divinity and metaphysics demands that claims connect with demonstrative or experiential evidence.