That's comprehensible and rational.
All of us as rational beings can't do anything else but live for our belly.
And all of a sudden the same Fyodor says that one mustn't live for one's belly, but must live for truth, for God, and at a hint I understand him! And I and millions of men, men who lived ages ago and men living now-- peasants, the poor in spirit and the learned, who have thought and written about it, in their obscure words saying the same thing--we are all agreed about this one thing: what we must live for and what is good.
I and all men have only one firm, incontestable, clear knowledge, and that knowledge cannot be explained by the reason--it is outside it, and has no causes and can have no effects.