The central theme of "No Longer Human" is the profound sense of alienation experienced by the protagonist, Oba Yozo. He feels disconnected from society, unable to relate to those around him, and struggles with a deep sense of isolation. This theme is explored through his interactions and the internal monologues that reveal his feelings of being an outsider.

Yozo's journey is marked by his struggle to understand his own identity. He grapples with the expectations placed upon him by society and his own perceptions of self-worth. This theme is crucial as it highlights the conflict between societal norms and personal identity, and the impact this has on one's mental health.

The book examines the clash between traditional Japanese values and Western influences. Yozo's experiences reflect the tension between these two cultural forces, and how they shape his worldview and personal struggles. This theme is significant as it provides insight into the broader cultural context of post-war Japan and the individual's place within it.