As Nero worsened, Seneca attempted to leave. Joining Nero’s administration was easy, but an exit was not. Nero could not afford to lose his most influential adviser, or allow the perception that someone as well known as Seneca was cutting ties with him. Seneca was granted a quiet sabbatical at Nero’s whim — the modern equivalent of a jointly issued news release.

Seneca had finally come to experience the truth of the words of the Roman poet Horace, whose work had greatly influenced him: “To have a great man for a friend seems pleasant to those who have never tried it; those who have, fear it.”

In a larger sense, Seneca’s struggle has echoes into our time, especially in politics. Last year, Senator Marco Rubio, a Republican who has both criticized Donald Trump and supported many of his policies, tweeted a quote from Seneca about tyranny, prompting some to ask if he was subtweeting the president. Ken Kurson, the former editor in chief of The New York Observer and an informal adviser to Mr. Trump and Jared Kushner during the election, told me that the Stoics were an inspiration to him as he dealt with the ethical and personal challenges of his position.

My own early career involved some questionable service to businesspeople. Employed and paid by them, I planned and carried out controversial publicity stunts, and used dishonest tactics with the public and the media. When I finally left those roles, I found a knowledge of Stoic philosophy integral to my ability to assess my past actions, and set a more honorable course going forward.

In a remarkable essay titled “On Leisure,” published after Seneca retired, the philosopher wrote in an oblique way about his own experiences: “The duty of a man is to be useful to his fellow-men; if possible, to be useful to many of them; failing this, to be useful to a few; failing this, to be useful to his neighbors, and, failing them, to himself: for when he helps others, he advances the general interests of mankind.”

Removed from the day-to-day of Rome’s geopolitics (helping the many), he seemed to have a newfound appreciation for helping the few. Seneca seemed to realize only belatedly that one can contribute to his fellow citizens in ways other than through the state — for instance, by writing or simply by being a good man at home. There is some irony in the fact that as an individual, the famous letters and essays Seneca wrote would not only have a bigger impact than his work in politics but also in time would whitewash his contributions to a horrible regime.

In 65 A.D., Seneca would again find that philosophy did not exist only in the ethereal world. Conspirators began to plot against Nero’s life, and Seneca, finally accepting that the monster he had helped create needed to be stopped, appears to have participated — or covered for those who did.