Human beings have two eyes, not because two·eyes necessarily see better than one, but because each sees from a slightly different angle.
Only thus is it possible to perceive distances and sizes.
Self-examination is like seeing with only one eye.
It doesn't really matter whether one looks at oneself from the ground up, seeing oneself as greater than one actually is, or from a the top downward, seeing oneself as smaller than one actually is.
In either case, the view is distorted.
Only the Supreme Judge, the Judge of the entire universe, is "the God of knowledge, and by Him actions are weighed" (I Samuel 2:3).
He can see everything from above, from below and from within, so only He can be a "sole judge" (Avot 4:8), while human beings – whoever they may be – cannot.
A human can see things from only one viewpoint: his own.
Therefore, in matters of the soul one should consult with others.
Not necessarily because the other person is wiser or more understanding, but simply because he has a different way of looking at things, and therefore his reasoning may be more balanced, more just.
This is why our Sages say that even the wisest of men needs an advisor – because seeing things from another angle gives the picture depth.
Even if one does not have an authoritative figure with whom to consult, one can seek out a close friend with whom he can share his deepest thoughts and feelings, and thus come as close as humanly possible to a just verdict.
–Rabbi Adin Steinsaltz
From an essay, "Self-examination" in A Dear Son to Me by Rabbi Adin Steinsaltz