In Russian, каменная стена (kamennaya stena), "stone wall" (our translation), is the word the narrator gives to the impossible: the laws of nature, the conclusions of the natural sciences, mathematics. Before it, men of strong nerves calm down. The narrator says he will not break the wall with his forehead, if he lacks the strength, but neither will he be reconciled to it just because his strength has failed him.
Where It Appears
Quotes
Of course I won't break through a wall like that with my forehead, if I really lack the strength to break through, but I also won't resign myself to it just because I have a stone wall before me and my strength was not enough.