Spots 
Mental contents divide into impressions and ideas, distinguished only by force and liveliness. Perceptions, sensations, and emotions strike the mind with immediate intensity as impressions. Ideas are faint images of these impressions retained in memory or recombined by imagination. A blind person possesses no concept of color because the original impression never occurred.
Thoughts rarely occur in isolation because a gentle force naturally connects them. Three distinct principles bind ideas together: similarity between objects, proximity in space or time, and cause and effect. When a mind holds one concept, these associative tracks guide it smoothly toward another without conscious decision. Complex thinking relies entirely on these habitual bonds rather than rational deduction.
Observing one event following another reveals contiguity and priority, but never a visible tie uniting them. One billiard ball strikes another, and motion follows, yet eyes detect no hidden force forcing the second to move. Repeated experience of constant conjunction creates a psychological expectation. Expectation, transformed into belief through custom, leads minds to project an inevitable bond onto external nature.
Introspection yields no enduring, simple substance called the self. Entering intimately into what passes for self reveals only particular perceptions of heat, cold, love, or grief in rapid succession. The mind acts as a theatre where diverse perceptions pass, glide away, and mingle in endless variation. Personal identity is a fictional continuity constructed by memory and associative relations.
Reason alone can never motivate an action or oppose a passion in the direction of the will. Demonstrative logic and empirical judgments merely map relations between objects or calculate means to an end. Desire or aversion provides the actual motive power that drives conduct. Rationality serves exclusively to inform emotions about the existence of objects and the methods to reach them.
Examining a willful murder reveals facts, relations, and circumstances, but no objective quality called vice. Vice escapes entirely until reflection turns inward to find a sentiment of disapprobation. Deductive systems shift silently from propositions linked by 'is' and 'is not' to claims joined by 'ought' and 'ought not'. This unseen transition introduces an entirely new relation that logic alone cannot justify.
Nature implants no instinctive, universal affection for public interest or distant strangers. Scarcity of external goods combined with human selfishness makes unconstrained interaction chaotic. Conventions arise gradually through mutual awareness of common interest, establishing rules of property and ownership. Justice becomes a virtue because these artificial rules stabilize society, earning moral approval through sympathy with the public good.
An ambitious attempt to build a naturalistic science of human mind, passion, and morality based on empirical observation.