Avos: 4:4 (sources)

Rabbi Levitas, a man of Yavneh, says: Be very, very humble in spirit, for the hope of man is worms: How can a man be proud at all - as in the end, the worms will be better than he? And about this he said, "Be very, very humble in spirit" - to emphasize the thing and say how great the punishment is for haughtiness and that is the proud one. And Rambam, may his memory be blessed, explained that he came to inform us that even though the middle point in all of the traits is the praiseworthy - like the trait of generosity, since both the spendthrift and the miser are bad and the middle is the [right] choice; and so with the trait of cruelty, that a person should not be cruel and [also] not completely merciful, as he should not have mercy on the wicked, and not be cruel to other people, rather the middle is the good path, to be merciful but to be cruel when needed; and so [too] totally with all (the intellect) [the traits], a man should grab the middle path and the moderate trait - yet this trait of haughtiness must be removed from oneself to the far extreme. As there is no trait more problematic than it; and most of the sins of the Torah depend upon it. And moreover, it causes forgetfulness of the Creator, may He be blessed, from the heart of a man - as it is stated (Deuteronomy 8:14), "And your heart grow haughty and you forget the Lord, your God." And this is what we learned [here], "Be very, very humble in spirit." And the sages of the Talmud have already argued about this thing in Tractate Sotah 5a - "One said, 'In excommunication is the one that has it and in excommunication is the one that does not have it at all.'" [This] is to say that a person should not be humble in spirit to the final extreme and not to be so lowly that people disparage him. Rather, he should be moderate in taking haughtiness - not to (beautify himself) [make himself proud], but not to lower his spirit to the utmost lowliness, that he not come to disgrace. And about this they said, "In excommunication is the one that has" much haughtiness "and in excommunication is the one that does not have it at all" - as he is not a person, but 'he is similar to the beasts.' "And the other one said, 'In excommunication is the one that has it at all'" - as it is so bad a trait that he must distance himself from it completely. And there should not be in him from it at all, like the opinion of Rabbi Levitas, and for the reason that we have written. And so is the law.

"secretly": And you [may] ask, behold, Rabbi Bartenura explained in Mishnah Yoma 8:8 that desecration of the Name (chillul Hashem) is [committed by] one who sins and causes others to sin; and this is one who sins and others learn from him; and if so, you don't find it "secretly," etc. And it appears to me that it is found with a sin that it is impossible [to do] without being joined by another - for example the sexual prohibitions. And it is also certainly true that one who eats forbidden foods and the like by himself and no one sees him is not [causing] a desecration of the Name at all. And see Tosafot Yom Tov.

"There is no differentiation between unintentional, etc.": It is known that the one who is unintentional also needs atonement. But he is not like the one who is intentional - God forbid for the straight paths of God, may He be blessed, to equate the intentional with the unintentional in anything. Rather, the intention [of the mishnah] is that his punishment is in public - if it was intentional, the punishment is [for an] intentional [sin]; if it was unintentional, the punishment is [for an] unintentional [sin], etc. And see Tosafot Yom Tov.