6 ideas
In the Great Unity age, 'the world was shared by all' (tianxia wei gong): rulers were chosen for worth and ability, and people did not treat only their own parents as parents or only their own children as children. Care reached the old, the young, widows, orphans, the childless and the disabled, so no category of person fell outside provision.
Because goods were not hoarded for private use and effort was not spent only for oneself, scheming did not arise and robbers and rebels did not appear. The outer doors were therefore left unlocked.
Once the Great Way was hidden, 'the world became the family's': people cared for their own kin, gathered goods and labor for themselves, and made rulership hereditary. Walls, moats and fortifications became necessary, and war and plotting arose from these private attachments.
In the Lesser Prosperity age, the sage-kings Yu, Tang, Wen, Wu, Cheng Wang and the Duke of Zhou used ritual and righteousness to fix the relations between ruler and minister, father and son, elder and younger brother, and husband and wife. Ritual is framed as a second-best technology: it does not restore the lost unity, but it restrains the self-interest that replaced it.
History is split into an ideal stage of public-mindedness and a lesser stage of stable private order. This gives reformers two targets: a realistic near goal of well-ordered moderate prosperity and a distant horizon of full commonality. Later thinkers reused the scheme as a stage theory of progress running forward toward datong, reversing its original backward-looking direction.
The passage defines the good society by concrete outcomes for those least able to fend for themselves: the aged live out their years, the able-bodied have work, the young are raised, and the helpless are supported. The test of a social order becomes what happens to the dependent, not how rulers are glorified or how strong the state is.