Patients suffering from schizophrenia are less susceptible to various perceptual illusions (and to some hallucinations, too) than most healthy individuals. Yet, schizophrenia patients’ perception-forming processes have been characterised as aberrant, as producing false inferences and irrational mental states. This characterisation is consistent with the idea that perceptual experiences and processes can be appraised as rational or irrational. But it remains unclear what norms should govern this appraisal, in either healthy individuals or psychiatric patients. In this paper, we consider various norms of Bayesian rationality, and argue that a violation of probabilism constitutes at least some cases of  irrationality of perception. Cases like schizophrenia patients’ resistance to illusions should be appraised as irrational, because it depends on a kind of probabilistic incoherence among perceptual experiences. This probabilistic incoherence manifests itself phenomenologically as fragmented perceptual scenes, whereby subjects of experience do not experience meaningful connections between the objects of their experiences.

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