When a person is alone, he sometimes talks to himself. Is he sick or mentally ill?

Of course it's not a disease, let alone a mental illness. Don't scare yourself if you have nothing to do!

Talking to yourself is very common, not a bad thing, and sometimes it just relieves mood swings.

Stay alone in a quiet room, trapped in a deep mountain cave ... no one talks to you at this time, which leads to emotional loneliness. You can get rid of this loneliness by talking to yourself.

Or in the wild, in order to calm down and scare off poisonous snakes and beasts, talking to yourself and shouting loudly can all play the role of courage and self-protection.

So, talking to yourself is normal. If there is a trance-like soliloquy, accompanied by panic and fear, it may be excessive shock and need to be channeled.