When three are walking together, I am sure to find teachers among them. I will select their good qualities and follow them, their bad qualities and avoid them.
子曰：“三人行，必有我师焉；择其善者而从之，其不善者而改之。” ----《论语·述而》
The teacher is the one who could propagate the doctrine，impart professional knowledge，and resolve doubts.
“传道受业解惑”. ---- 韩愈《师说》