6 ideas
We cannot feel another's sensations directly, so we judge their feelings by imagining ourselves in their situation and checking whether our reproduced emotion matches theirs. Approval is simply the perception of this correspondence, and disapproval is the felt gap between what they show and what we can go along with.
Conscience is an internalized observer, built up from repeatedly seeing how others judge us and imagining how a well-informed, disinterested bystander would view our conduct. By viewing ourselves 'at a certain distance' through this figure's eyes, we correct the huge distortion self-love introduces, so our own small interests stop outweighing the great interests of others.
In the heat of acting, passion distorts our view of our own conduct, and afterward we avoid honestly reviewing it because the verdict would be painful. This motivated blindness is 'the source of half the disorders of human life', and the remedy is general rules of conduct formed in calm moments from observing others, which hold us steady when self-assessment fails.
A poor man's ambitious son, dazzled by the conveniences of the rich, spends his life in exhausting toil to acquire them, only to find in old age that wealth and greatness are 'trinkets of frivolous utility' that bring little real tranquillity. Yet this deception is useful, because it drives the industry that cultivates land, builds cities and advances the arts for everyone.
The rich, pursuing only their own vanity and convenience, can eat little more than the poor, so they must employ thousands to produce their luxuries and thereby share the earth's produce. They are 'led by an invisible hand' to make nearly the same distribution of necessaries as would have occurred had the earth been divided equally, advancing society's interest without intending it.
The 'man of system' treats members of society like chess pieces he can arrange at will, forgetting that each piece has its own principle of motion that may differ from what the planner imposes. Reform fails or produces disorder when these independent motions oppose the design, so wise statesmen adapt plans to people's existing habits and prejudices rather than forcing an ideal blueprint.