A person imagining a person talking to themselves may be a form of mental illness, but it may also be part of a normal thought process. Here are some possible explanations:

1. Schizophrenia: Schizophrenia is a mental illness in which hallucinations and delusions are a symptom. In this case, a person may imagine a person having a conversation with him or her, while being convinced that the conversation is real and not imaginary.

2. Obsessive-compulsive thoughts: Obsessive-compulsive thoughts are an anxiety disorder in which a person may have obsessive thoughts, that is, imagining a person talking to themselves and being unable to get rid of this thought.

3. Self-talk: Self-talk is a normal thinking process. Everyone talks to themselves when thinking about problems. This self-talk is usually short-lived and has no impact on daily life.

So if a person only occasionally imagines a person talking to them, and there are no other signs of mental illness, this may be normal. But if the imaginary conversations persist, interfere with daily life, or are accompanied by other signs of mental illness, it is recommended to seek professional medical advice to determine diagnosis and treatment options.