Theoretisch gibt es eine vollkommene Glücksmöglichkeit: An das Unzerstörbare in sich glauben und nicht zu ihm streben.
Theoretically there is a perfect possibility of happiness: believing in the indestructable element in oneself and not striving towards it. [Kaiser/Wilkins]
Theoretically, there is one consummate possibility of felicity: to believe in the indestructability in oneself, and then not to go looking for it. [Hofmann]
Theoretically, which is to say not only that Kafka does not claim to believe this himself, but that he is only willing to grant that it is provisionally possible. Kafka cannot fail to detect any gulf between theory and practice.
Believing there is something in you that cannot be destroyed, rather than trying to achieve a measure of indestructability, is happiness, even perfect happiness. Not immortality; he says indestructability. Immortality is an existence without death, whereas a indestructable thing may meet with deadly adversity, but it shrugs it off or survives it. That happiness isn't neverending life, but confidence.