Pirkei Avot Chapter 1, Mishnah 2 reads:
1:2 – Simon the Righteous was one of the last survivors of the Great Assembly. He used to say: On three things the world is sustained: on the Torah, on the (Temple) service, and on deeds of loving kindness.
Who was Simon the Righteous (Shimon HaTzadik)? Aside for being one of the last survivors of the Great Assembly, he was also the High Priest.
According to a famous story in the Talmud (Yomah 69a), Simon went out to meet Alexander the Great, who the Samaritans were hoping would destroy the Temple. Simon donned his priestly clothing for the occasion, and when Alexander saw him, he got down from his horse and bowed down to Simon.
Says the Talmud:
They said to him: Will such a great king as you bow to that Jew? He replied: His image I saw shining before me, whenever I gained a victory. He asked the Jews: Wherefore are you come? They said: The Temple where we pray for you, and for your empire, that it should not be destroyed, is it possible that you should be misled by the idolaters to bid its destruction? He asked: Who are those idolaters? They replied: These Samaritans who stand near thee. He said to them: I deliver them into your hands.
This story is likely apocryphal, of course. Simon probably lived some time between 310 and 270 BCE or from 219-198 BCE (depending on which Simon he actually is). Alexander the Great died in 323 BCE.
For Pirkei Avot, most of this is irrelevant. What is important: As a key to the chain of tradition, Simon is something of a proto-Rabbi, who not only speaks in aphorisms, but names Torah as the first of the three things that sustain the world.
But he is not stripped of his priestly background. “Avodah,” the Temple service, is — according to Simon — the second item that sustains the world. This could be a difficult notion for Pirkei Avot. After all, the Rabbis of Pirkei Avot are trying to assert their post-Temple control on Judaism. They are the spiritual descendants of the Pharisees, the opponents of the Sadducees, who were of the priestly class.