A young student befriends an older man in Tokyo. The older one's intellectual abilities, and his sophistication gains him the title of 'Sensei' - roughly approximating 'teacher' or 'master' - from the younger one.
Though he likes him well enough, Sensei does nothing to encourage the young man's growing attachment to him. This only increases the student's interest in Sensei's life, who responds finally to his overtures of friendship and respect thus: 'I do not want your admiration now, because I do not want your insults in the future. I bear with my loneliness now in order to avoid greater loneliness in the years ahead. You see, loneliness is the price we have to pay for being born in this modern age, so full of freedom, independence, and our own egotistical selves'.
The novel is structured in three parts. The first two are narrated by the student, and the third is a 'testament' in letter form by Sensei, outlining the story of his life, and explaining why he has for so long withdrawn from the outside world.
Sensei's testament is a profound self-examination and self-criticism, mostly revolving around his selfish and manipulative actions, in his own student days, when he and his friend (a fellow student) were both in love with the same girl (now Sensei's wife). This behaviour leads, in the end, to catastrophic results for his friend. From that period on, though Sensei has appeared outwardly normal and happy, his life has been completely blighted.
What makes the novel such a significant work for Western readers (other than its literary excellence) is the distinctly Japanese point of view it brings to an old story. This new perspective brings up a large number of worrying (because unanswerable) questions. How much, for instance, does Sensei's failure to forgive himself for his earlier mistakes arise from his culture's sense of 'honour', and how much from human nature?
Kokoro translates as 'the heart of things', a perfect title for a book that delicately, subtly and finally disturbingly, probes the mystery that is the core of human life.