Thought insertion is the idea that another thinks through the mind of the person. The person may sometimes be unable to distinguish between their own thoughts and those inserted into their minds. A person with this delusional belief is found to be convinced of their beliefs and unwilling to accept such diagnosis. It is a symptom of psychosis which occurs in many mental disorders and other medical conditions. Thought insertion along with thought broadcasting, thought withdrawal, thought blocking as well as other first rank symptoms are primary symptoms and should not be confused with the delusional explanation given by the respondent.
The ICD-10 defines thought insertion as the feeling that your thoughts are not your own. The person experiencing the thought insertion will not necessarily know where the thought is coming from. 
- Xavier Francisco Amador; Anthony S. David (2004). Insight and psychosis: awareness of illness in schizophrenia and related disorders. Oxford University Press. pp. 67–69. ISBN 978-0-19-852568-4. Retrieved 12 December 2010.
- World Health Organization (January 1992). The ICD-10 classification of mental and behavioural disorders: clinical descriptions and diagnostic guidelines. World Health Organization. ISBN 978-92-4-154422-1. Retrieved 16 December 2010.