expression
it must be like this; surely it is so
Archaic or highly literary expression conveying strong conjecture or rhetorical affirmation: 'this is how it must be' or 'surely it is so'. Not used in modern everyday speech.
古の賢人も斯くやあらんと感じたに違いない。
The ancient sages must have felt that it was just like this.
「斯くやあらん」は古典で見かける言い回しだ。
The phrase 'kaku ya aran' is an expression one encounters in classical literature.