A person knows his own soul.
A person is well aware of his feelings and thoughts.
("And from this wisdom and understanding, a man becomes wise in the lowliness of his power and the lowliness of his worth, to be a lion, calculating how far he is from God, and when a man knows in his soul, then he is despised and is utterly destroyed in his own eyes, and from this a terror falls upon him and he fears that he will not be granted mercy and peace." The Holy of Holies (Levi's Words)
Translation
Expression (Hebrew)
יָדַע אִינֵּשׁ בְּנַפְשׁוֹ
Definition (Hebrew)
אָדָם מוּדָע הֵיטֵב לְרִגְשׁוֹתָיו וּמַחְשְׁבוֹתָיו.
("וּמֵהִסְתַּכְּלוּת וְהִתְבּוֹנְנוּת זֹאת מִסְתַּעֵף לִהְיוֹת מִסְתַּכֵּל עַל שִׁפְלוּת עַצְמוּתוֹ וּפְחִיתוּת עָרְכוּ, לִהְיוֹת מִמַּאֲרִיָּה דְּחוּשְׁבָּנָא אֵיךְ שֶׁהוּא רָחוֹק מֶה', וְכַאֲשֶׁר יֶדַע אִנֵּשׁ בְּנַפְשׁוֹ, אָז הוּא נִבְזֶה וְנִמְאַס בְּעֵינָיו בְּתַכְלִית, וּמִזֶּה תִּפּוֹל עָלָיו אֵימָה וְיִרְאָה שֶׁלֹּא לַעֲבֹר עַל רְצוֹנוֹ חַס וְשָׁלוֹם"- קְדֻשַּׁת לֵוִי דְּבָרִים פָּרָשַׁת עֵקֶב)