PIRKEI AVOT 1:16

 Rabban Gamaliel says: Provide yourself with a teacher and remove yourself from doubt, and do not accustom yourself to give tithes by estimation.

The Mishnah says that one should make for oneself a “Rav,” stay away from doubt, and not accustom ourselves to tithe imprecisely.
Although one could say that these three admonitions appear together only because they were all said by the same person (Rabban Gamliel), I would like to suggest that they are thematically linked as well.
The basic idea is that when all of one’s decisions are made in consultation with one’s personal Rav, doubt will not exist. When would it happen that one will be forced to “act according to estimation?” That lack of certainty occurs when one has not designated for oneself a Rav.
When one has a personal Rav, epx becomes ex p. (Sa-fek [doubt] becomes pe-sak).
The Rabbis in Massechet Berachot tell us something similar: Whoever takes benefit from this world without offering a blessing beforehand, has sinned.
(He is considered as if he had taken from that which is kodesh and used it for something personal and mundane, which is called me’ilah).
The gemara continues and asks, “What’s his remedy?” It answers, “He should go to a Chacham.” To this, the gemara protests: “But this sin is already accomplished; what good will it do to go to the Chacham at this point?” The answer: “Let him go to the Chacham beforehand -- and have him teach him the laws of berachot! That way, he’ll no longer come to sin!” In other words, the trip to the Chacham will free him of sefeikot. (For, he will know the correct blessing).