ghost is a very ancient notion in mankind history and you can find reference to it in lots of ancient civilisation. so what is a ghost, it is always referenced as what remind of you when your body or your physical existence is not there anymore, and i guess it was first used for leaders, or charismatic person, who gone dead and that his will, or action are to be followed or thougth of after their death, like a remain of what he was, and its like 'it still exist in our mind' , or when somebody wanted to accomplish something that he could not finish in his life and that other person want to be done following his will =)
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ghost are the first concept that gave the consciousness of what we do in our life,and what become of us after we die, and well i think it more or less is the root of any religion to believe in such things that what you do in your life can go beyond the moment you die, even if you are not physically present in this world any more. and that 'dead spirit' can eventually connect our mind by some manner and dead spirit can still be present in this world after they die by the mean of our own mind.
just a thougth lol hope it doesn't sound to much glaucous or morbid or scary