משנה אבות ו-ז(רוני,שוהם,אביב)

(6) Yehoshua ben Perachiah and Nitai of Arbel received from them. Yehoshua ben Perachia says, "Make for yourself a mentor, acquire for yourself a friend and judge every person as meritorious."

(1) "Make for yourself a (Torah) mentor": Rambam explained, “Even though he is not fit to be your teacher, make him into your teacher and do not learn on your own.” And I heard, “'Make for yourself a teacher,' [meaning] that he should take on one teacher from which always to learn – and not learn from one today and from another one tomorrow." And even though they stated in tractate Avodah Zara 19, “One who learns from [only] one teacher will not see a sign of blessing;” they have already explained and said, “this applies to reasoning” - as it is good for him to hear the reasoning of the many - “but with concern to [memorization of traditional teachings], it is better from one teacher, so that [the student's] elocution not be damaged."

(2) "acquire for yourself a friend": And even if you need to acquire him for a large sum and to spend money upon him in order to acquire his love. But with a mentor, it is not applicable to say, "Acquire for yourself a mentor," as a [teacher of Torah] must teach for free.

(3) "and judge every person as meritorious": when the matter is hanging in the balance and there is no way to decide it in this way or that way. For example, a man from whose actions we do not know if he is righteous or wicked, who preforms an act that is possible to judge favorably and possible to judge unfavorably, it is pious to judge him favorably. But it is permissible to judge a man who is established to be evil unfavorably. As they only stated (Shabbat 97), “one who suspects righteous people is afflicted on his body” - [and so] it is implied [from this] that one who suspects evildoers is not afflicted.