They are, if we can cannot even explain consciousness what makes anyone think that they have a clear cut answer to what happens to the spirit/soul/person when they die. I am a scientist by nature and studies and I see more evidence to the fact they do exist. In what context they exist, who knows?
I have experienced some ghosts in subconscious state. Once a ghost tried to smother me. Once I found a lady ghost lying beside me. I even touched her long and smooth hairs. Some ghosts are funny. They call you by your name or they shout, "Wake up! there is a thief inside your house." Many times they wake me up in early morning by touching gently in my head. I don't know whether these experiences are real or not.
Yes ,I have witnessed many ghost,just recently six weeks ago. I started a 24hr.home care position for a friend's mother ,and every one of the mirrors in her house were covered up,even the bathroom ones, and I asked why and the daughter told me that her mom has been seeing someone,and that the Dr. had put he on medication,and suggested it,some how relating it with her Alzheimer's.It was about 11:00 watching T.V. and heard a rattle coming from the kitchen,I knew Maria went to bed,then something ran through the kitchen and the door alarm went off,Maria woke up and while I was setting the door alarm Maria tell's me this lady thinks this is her house and I tell her this is my house,my house,and I try to be nice to her,so me and Maria held hands and asked Jesus to place any lost souls where thay needed to be,,and then Maria say's but we still friends,and that was it,.I told her children what had happened,over the weekend and over the next few weeks,no sightings,so about half of the mirrors were uncovered,and she was taken off her meds.
Given our society's current interest in "supernatural" phenomena, this is an important question. Some people reject the idea of ghosts as crazy or superstitious, putting them on the same level as werewolves, vampires, or extraterrestrials. But let's take a look at what Catholic teaching has to say on the matter. Before we can ask whether ghosts exist, we need to be clear on what we're talking about. A "ghost," according to the definition found in Webster's New Collegiate dictionary, is a "the soul of a dead person, a disembodied human spirit." So a ghost is NOT an angel or a demon; rather, it is that part of a human being. - the soul or spirit - which is separated from the body at death. So everyone now in heaven, hell, or purgatory could be called a "ghost," and this will be the case until the Last Judgment, when our souls will be reunited with our bodies. In this sense, we as Catholics affirm that "ghosts" exist. (My answer continues in the comments)