Deficits of logical reasoning have long been considered a hallmark of schizophrenia and delusional disorders. We provide a more precise characterization of 'logic' and, by extension, of 'deficits in logical reasoning.' A model is offered to categorize different forms of logical deficits. This model acknowledges not only problems with making inferences, which is how logic deficits are usually conceived, but also problems in the acquisition and evaluation of premises (i.e., filtering of 'input'). Early (1940-1969) and modern (1970-present) literature on logical reasoning and schizophrenia is evaluated within the context of the presented model. We argue that, despite a substantial history of interest in the topic, research to date has been inconclusive on the fundamental question of whether patients with delusional ideation show abnormalities in logical reasoning. This may be due to heterogeneous definitions of 'logic,' variability in the composition of patient samples, and floor effects among the healthy controls. In spite of these difficulties, the available evidence suggests that deficits in logical reasoning are more likely to occur due to faulty assessment of premises than to a defect in the structure of inferences. Such deficits seem to be provoked (in healthy individuals) or exacerbated (in patients with schizophrenia) by emotional content. The hypothesis is offered that delusional ideation is primarily affect-driven, and that a mechanism present in healthy individuals when they are emotionally challenged may be inappropriately activated in patients who are delusional.