4 ideas
Understanding comes from probing the principle of one concrete thing today and another tomorrow, not from grasping everything at once. Sources include reading, discussing people past and present, and handling affairs well. Once enough cases have accumulated, a sudden integrated understanding emerges in which the connections between things become clear.
Self-cultivation rests on seriousness (jing): keeping the mind collected on one thing without letting it wander, and keeping one's outward demeanor orderly and grave. This steady attentiveness is paired with extending knowledge. Without it, study cannot take hold, because a scattered mind cannot hold or test what it learns.
There is a single underlying principle, and it shows up as different obligations depending on one's position and relationships. Seeing all people as siblings and all things as companions gives a universal basis for care. That care is still properly differentiated by one's specific ties, like the special duties owed to parents, so universality does not collapse into undifferentiated love.
As a young man Cheng Hao loved hunting and later believed he had outgrown the desire, but a teacher warned that it was only lying dormant. Twelve years later he saw hunters in a field and felt a spark of delight, which proved the teacher right. The lesson is that a desire that has gone quiet is not the same as a desire that has been transformed, and self-examination has to probe below apparent calm.