I believe in ghosts. I used to be in one of those ghost tracker units..
Being able to see into the higher plane (4th dimension/astral plane) is called clairvoyance. Many are gifted with this "extra sense" (its more like an ability that fell into disuse) and it's an ability you can develop with education and practice.
Some spirits seem to be able to cross into our plane partially. I can't explain how they do this.