Can people experience things that are not there?

There are people, who sometimes claim they have seen, or otherwise experienced matters that the normal person may not be able to accept as true. It has various description with different size, shapes and colours with unique narrations. This may be due to an illusion or delusion or hallucination.

One that you see does not exists

An illusion is an error of sense perception in which an individual misinterprets something that he actually experiences. In the case of delusion, it is a faulty belief peculiar to the person concerned.

The other me that I don't know

Hallucinations are false sensory perceptions in which the individual experiences something which others agree is simply not there. Hallucination may be visual auditory or related to the senses of smell or touch.

Persons who are hallucinating are usually in a high state of excitement, fear, ecstasy, or anticipation of something. Even the ancients knew that certain plant juices could induce hallucinations. A modern counterpart is a group of drugs, tranquillizers.