To the reply that said that there were extra-terrestrial in addition to extra-dimensional... The whole concept is that the supposed aliens are from this planet. The only difference is that they exist on a totally different frequency of existence, than us. As such it can be applied to ghosts as well...Witches, well some of us are known as talented mediums, and we can see past our frequency, thus see ghosts. Others see aliens, this is because both entities are in different frequencies. Watch the movie stargate and think of it as travelling to different frequencies. The theory of dimensional frequency is a very hard one to grasp, and is very complex to explain.