A fool thinks everyone is a fool/ a fool believes that everybody else is one too
A fool thinks everyone is foolish. He does not realize that he is the odd one out in his foolishness.
("And also on the road, when the fool goes along... the fool thought that all people were fools like him, and he did not know that he was the fool and all people were wise" - Kohelet Rabbah (Vilna), Parashah 10, 3)
Translation
Expression (Hebrew)
טִיפֵּשׁ חוֹשֵׁב שֶׁכָּל הָעָם טִפֵּשׁ
Definition (Hebrew)
הַטִּיפֵּשׁ חוֹשֵׁב שֶׁכֻּולָּם טִיפְּשִׁים. הַטִּיפֵּשׁ אֵינוֹ יוֹדֵעַ שֶׁהוּא הֶחָרִיג בְּטִפְּשׁוּתוֹ.
("וְגַם בַּדֶּרֶךְ כְּשֶׁהַסָּכָל הוֹלֵךְ וְגֵו', טִפְשָׁא סָבַר דְּכָל עַמָּא טַפְּשִׁין כַּוּוֹתֶיהָ וְהוּא לֹא יָדַע דְּהוּא טַפְשָׁא וְכָל עַמָּא חַכִּימִין"- קֹהֶלֶת רַבָּה (וִילְנָא) פָּרָשָׁה י א ג)