וָאֶתְּנָ֤ה לִבִּי֙ לָדַ֣עַת חָכְמָ֔ה וְדַ֥עַת הוֹלֵל֖וֹת וְשִׂכְל֑וּת יָדַ֕עְתִּי שֶׁגַּם־זֶ֥ה ה֖וּא רַעְי֥וֹן רֽוּחַ׃
And I applied my heart to know wisdom, and to know madness and folly—I perceived that this also was a striving after wind.
Rashi on Ecclesiastes
And I applied my heart, now, to know the nature of wisdom, what its end is, and the nature of madness and folly.
Rashi on Ecclesiastes
Madness. Dullness and confusion of thoughts, an expression of mixing, as in, “diluted מָהוּל with water.”49Yeshayahu 1:22.
Rashi on Ecclesiastes
And folly. Foolishness.50שׂכלות has the same meaning as if it were spelled סכלות [=foolishness], as in, “and to grasp foolishness [סכלות]” in 2:3 and as in, “and madness and foolishness [סכלות]” in 2:12 below.