Thought insertion is defined by the ICD-10 as the delusion that one's thoughts are not one's own, but rather belong to someone else and have been inserted into one's mind. The person experiencing the thought insertion delusion will not necessarily know where the thought is coming from, but makes a distinction between their own thoughts and those inserted into their minds. However, patients do not experience all thoughts as inserted; only certain ones, normally following a similar content or pattern. A person with this delusional belief is convinced of the veracity of their beliefs and is unwilling to accept such diagnosis.

Thought insertion is a common symptom of psychosis and occurs in many mental disorders and other medical conditions. However, thought insertion is most commonly associated with schizophrenia. Thought insertion, along with thought broadcasting, thought withdrawal, thought blocking and other first rank symptoms, is a primary symptom and should not be confused with the delusional explanation given by the respondent. Although normally associated with some form of psychopathology, thought insertion can also be experienced in those considered nonpathological, usually in spiritual contexts, but also in culturally influenced practices such as mediumship and automatic writing.

Some patients have also stated that at some point in time they were being manipulated by an exterior or interior force (depending on the delusion that the patient faced) and only later realized that thoughts weren't theirs; this is linked to patients "losing control" of what they do.^{[citation needed]}

Unbidden thoughts are thoughts that come to mind without the subject directly or consciously meaning to think them. Inserted thoughts, while sharing the characteristic of unconsciously or indirectly being conjured, are distinct from unbidden thoughts because of the author of the thoughts. During an experience of unbidden thinking, the subject knows that they are the author of the thought even though they may not have consciously meant to think it. During the feeling of thought insertion, however, the subject feels as though the thought belongs to another person and was inserted into their own mind, making the author of the thought someone other than themselves.^{[citation needed]}

Auditory hallucinations have two essential components: audibility and alienation. This differentiates it from thought insertion. While auditory hallucination does share the experience of alienation (patients cannot recognize that the thoughts they are having are self-generated), thought insertion lacks the audibility component (experiencing the thoughts as occurring outside of their mind or spoken to them). Thus the person experiencing thought insertion recognizes that the thought is being thought of inside their mind, but fail to recognize they are the one thinking it.

Examples of thought insertion:

She said that sometimes it seemed to be her own thought 'but I don't get the feeling that it is'. She said her 'own thoughts might say the same thing', 'but the feeling isn't the same', 'the feeling is that it is somebody else's.'

I look out the window and I think that the garden looks nice and the grass looks cool, but the thoughts of Eamonn Andrews come into my mind. There are no other thoughts there, only his. He treats my mind like a screen and flashes thoughts onto it like you flash a picture.