Thought is a mental activity by which people reason, solve problems, form judgements and communicate with each other by speech, writing and behaviour. Disturbances of thought are reflected in how a person communicates: the normal logic of thought is broken up and a person may randomly move from one subject to another. SCHIZOPHRENIA is a mental illness characterised by thought disorder. Confusion, DEMENTIA, DEPRESSION and MANIA are other conditions in which thought disorders may be a marked feature. (See also MENTAL ILLNESS.)