People with frontal lobe damage often have few and poor beliefs. An important part of the cause of this is their loss of ability for counterfactual thinking. Occasionally a patient with epilepsy arising in the medial temporal lobe reports abnormally intense belief that is free-floating and attaches to everything in consciousness. A similar intense belief sometimes occurs under the influence of mindaltering drugs, and during supreme athletic moments. Odd effects on belief occur in neurological neglect syndromes, and after surgical section of the corpus callosum. There are striking effects on belief in some patients with mental illness, including depression, obsessional hypochondriasis, and obsessive-compulsive disorder. Schizophrenia impinges on belief in several ways. Delusions, reality testing, and empathy are examined, and the possibility that belief is unnaturally intense in some sufferers is raised.