Hypothetical: You are an agnostic ghost. As a ghost, you cannot interact with physical objects or people, with one exception: when you are near a person you knew when alive as they are thinking about you, you can hear their thoughts and communicate to them by imparting a thought into them - making it pop into their head mimicking one of their own thoughts. They cannot know you are a ghost or see you. Your biggest regret is a fight you had with a friend before you died which has meant they no longer think kindly of you. You have observed that once a ghost resolves their biggest earthly regret, they disappear. If you do not, you remain a ghost for perpetuity.