My think its like this.
The body is made up of...mind...body....and soul.... The body is returned to the land..... The soul is returned to the creator of life.....and your mind will live on.... So all in all i say that the spirits that you see are the minds of the non-living projecting themselfs into you mind....
All in all i dont beleive it is "satan" tricking your mind.

Ok..if the images we see of loved ones that have passed are "demons" how could i have been talking with my grandma and best friend who died? i believe that some will be garudian angels and be there when you need to talk and some might be demons and as a christian i know there is a demonic presence within all spirits.....just my opinion

Ever seen the grudge? Or have u read up on evil spirits if the dead? The ancent greeks believed if a body wasent buried properly their spirit would haunt the living. There's also a myth in some curtures about the idea of people dying horrible deads and coming back not knowing why they r still here and wonder the earth not knowing they died. The truth of the matter to me is death may not be the end and perhaps we will never know but ik demons r real and ghosts could be what happens wen some people cannot rest after they r dead. Like socrotes said all souls are immortal the souls of the rightious are both immortal and divine

@Donnie1984: This may sound strange since I am pagan but I believe some spirits are people that have passed on and others are demons. So I believe it could be either. However demons are much more malicious than human or animal spirits.