The law of the valuation of a person is connected to the sanctity of speech – "this is the thing" referring to speech – as the Sefer HaChinuch states:
Because a person only partakes with the heavenly beings through speech, and this is the most honorable part of him, and this is called in man 'a living soul,' as Onkelos translates, 'and it became in man a speaking spirit,' for the other parts of the body are dead, and if a person loses this good part, the body remains dead and like a worthless vessel. Therefore, one is obligated to uphold his word in matters related to the words of Heaven in any case, such as consecrations and all matters of charity. And in all other matters of the world... the sages commanded and warned with many warnings that a person should not change his word.
Caution in the attribute of truth is related to the essence of man-the-speaker; in this, he is similar to God, and therefore, one must not alter his speech, in the manner of "I the Lord do not change" (I do not change My word), "distance yourself from a false matter," which is the foundation of the entire Torah, "Moses is true, and his Torah is true," and it is also the foundation of educating the young (as stated by the Shelah), because the letter alef’s name, ףֶלָא, is an acronym for “teach your mouth” (אמֶתָ למֵדִ פָּיך).
The attribute of truth sweetens the curses, which are also through the power of speech, "this is the thing," as mentioned above.
It is told that when Rabbi Hillel of Paritch wanted to get closer to the Alter Rebbe, he prepared a question on Tractate Arakhin. He went to the room where the Alter Rebbe was supposed to stay and hid under the bed. When the Alter Rebbe entered the room, he said to himself in a melodious tone, "A young man who has a question on Tractate Arakhin, should first evaluate himself." Rabbi Hillel fainted, and the Alter Rebbe continued on his way. The internal breaking that occurred in Rabbi Hillel touches upon the attribute of truth, to evaluate oneself without deceit, and thus attain humility like Moses our teacher.
(translated form Pri Eitz on Bechukotai)
