Adapting a term coined by Ian Hacking, this article analyzes certain of the styles of reasoning that appear in two novels, Fyodor Dostoevsky's Zapiski iz podpol'ya (Notes from Underground, 1864) and Natsume Sōseki's Kokoro (1914). The confession of the underground man, the protagonist of Dostoevsky's novel, includes an argument against the Chernyshevskian doctrine of rational egoism. The underground man's argument may, as this article shows, be analyzed using logical truth tables to demonstrate that, however thorough the underground man's argument may appear, it does not consider the counterexample of selfless altruism. This omission prepares the way for the underground man's rejection of Liza at the climax of the second part of the novel. Sōseki's novel, too, contains a confession, namely Sensei's testament, in which Sensei relates how he arrived at his belief that humanity is fundamentally selfish. Sensei's style of reasoning is primarily inductive, in contrast with that of other characters in Sōseki's novel, and the present article argues that Sensei's style of reasoning is a primary cause of his suicide. In each novel, then, there is a sustained consideration of how and to what extent a style of reasoning is bound up with a character's fate.