What did Marcus Aurelius learn from his adoptive father Antoninus Pius?

Marcus does not describe Antoninus in abstractions. He watches him, and Meditations 1.16 is a long inventory of specific, observed behavior, the kind of evidence a student of character actually needs.

The entry opens with mildness of temper paired with unshakeable resolve once a decision has been made through due deliberation (Meditations 1.16). That combination matters. Marcus is not praising a soft man or a stubborn one. He is describing someone whose calm and his firmness come from the same source: careful thought that has already been done, so there is nothing left to be anxious or defensive about.

Much of the passage is about how Antoninus handled power without being changed by it. He took no vainglory in the things men call honours, considered himself no more than any other citizen, and released his friends from any obligation to attend him (Meditations 1.16). He checked popular applause and flattery immediately, was not superstitious about the gods, and did not try to win people over with gifts or by flattering the populace (Meditations 1.16). This is the common misreading worth correcting: people sometimes imagine Stoic composure as a kind of emotional flatness. What Marcus actually records is a man who enjoyed the comforts fortune gave him without arrogance, and did not miss them when they were absent, so that he used them without affectation, and when he had them not, he did not want them (Meditations 1.16). That is not suppression. It is indifference in the technical sense: not caring more about externals than they deserve, while still being fully present to ordinary life, easy in conversation, and agreeable without any offensive affectation (Meditations 1.16).

Marcus also notices Antoninus's method, not just his temperament. He describes a habit of careful inquiry in all matters of deliberation, and persistency, and that he never stopped his investigation through being satisfied with appearances which first present themselves (Meditations 1.16). In other words, Antoninus treated first impressions as a starting point for judgment, not a verdict. He kept digging until he had reason to stop. Marcus pairs this with undeviating firmness in giving to every man according to his deserts (Meditations 1.16), so the inquiry was in service of fairness, not mere caution.

There is a public dimension too. Antoninus was watchful over what the administration of the empire required, a good manager of expenditure, and patiently endured the blame he got for such conduct (Meditations 1.16). He looked to what ought to be done, not to the reputation which is got by a man's acts (Meditations 1.16). That line separates two things people often collapse into one: doing right, and being seen to do right. Antoninus aimed at the first and let the second take care of itself, sometimes at his own cost.

What changes for a reader who takes this seriously is the target of admiration. Marcus is not holding up Antoninus's achievements, his office, or his reputation. He is holding up a pattern of small, repeated choices: not tiring of friends without being extravagant in affection, taking a reasonable care of his body without vanity or neglect, staying in the same places and the same occupations rather than chasing novelty (Meditations 1.16). None of this is dramatic. That is the point. Marcus even closes the portrait with an ordinary anecdote, how Antoninus behaved to the toll-collector at Tusculum who asked his pardon, and adds simply that such was all his behaviour (Meditations 1.16).

The concrete practice this suggests is not complicated: pick one trait from this list, such as checking your own move toward flattery or applause, and watch for it in yourself for a day, the way Marcus watched Antoninus for years.