The article concerns the Stefan Themerson’s novel The mystery of the sardine (polish title: Euklides był osłem). First, it discusses the philosophical inspirations inherent in the novel. Then a critique of the mental tendencies in Western philosophy symbolized by Euclid is presented. Themerson criticizes the assumption, that eternal and immutable things are the basis of mutable and temporal things. He also rejects the view, that a priori knowledge about the real world is possible. Instead, he proclaims the primacy of factual truths over those of reason, and the primacy of experience over abstract inferences. He also argues that assigning too much of a priori cognition leads to the creation of world repair projects, which justify fanaticism and cruelty. Instead, he proposes an ethics of good manners resp. rules of decency.