If you were to destroy in mankind the belief in immortality, not only love but every living force maintaining the life of the world would at once be dried up. Moreover, nothing then would be immoral, everything would be lawful, even cannibalism. The Karamazov Brothers

As surgeons keep their instruments and knives always at hand for cases requiring immediate treatment, so shouldst thou have thy thoughts ready to understand things divine and human, remembering in thy every act, even the smallest, how close is the bond that unites the two. Book III

“I wanted to excel in your eyes, show my strength of will, wait before writing you, first finish a document, but the room is empty, no one is minding me – it’s as if someone said: leave him alone, can’t you see how engrossed he is in his own affairs, it’s as if he had a fist in his mouth. So I only wrote half a page and am once again with you, lying on this letter like I lay next to you back then in the forest.” (16 July 1920) Letters To Milena

Not to live as if you had endless years ahead of you. Death overshadows you. While you're alive and able—be good. Book IV