Thematically, the novel is primarily about the nature of memory.

In plot terms, the driving force of the novel is the Professor’s inability to remember anything from more than 80 minutes prior—this is the reason he needs a housekeeper, the reason his sister-in-law must care for him financially, and the primary obstacle that the narrator and Root face in developing a personal relationship with him.

However, memory is complex: Ogawa separates intelligence from neurological conditions or disabilities. The Professor’s memory cannot be reduced to his internal cognitive function—though the Professor cannot perform certain daily functions, he still retains his brilliance. Most obviously, The Professor’s inability to remember what happened the day before does not deter him from working on complex mathematical proofs though this must be incredibly difficult for him, as he has to begin anew each day. Nevertheless, he still wins frequent, impressive prizes for his work.

Moreover, memory exists in other forms. The Professor wears suits covered in notes to help him remember the most important things. While this reinforces the limitations of such systems when compared to the power of our minds, it also suggests something about the value of memories.