Great concept! Some theories i know go like this: When our bodies die the energies left from our thought patterns and emotional vibes still remain- eventually these fade (unless you're someone famous like marilyn munroe and people keep giving energy to that energy). Some psychics etc. Pick up on that. Another thought is that our true selves, our soul nature is alive and well and can be connected with this is the most fascinating thought with your query because indeed the dead are dead and when we communicate with the essence of 'many aunt doris' we are in fact communicating with her true eternal self the self that cannot die which invaribly would not be many or old at all!. And lastly that a psychic or intuitive person can 'read' the energy of the person they are sitting with (it's a bit like when you know the phone going to ring before it does..). Hope thats clear for you.
It is invalid because it uses two definitions of the word "dead". "The dead" refers to someone whose body has died, though their soul may still be alive. "The dead" are not physically or bodily alive, but if there is a soul that survives after the body dies, then they can be considered to be "alive" in another sense.
It's the fallacy of equivocation, I believe. The word "dead" in the first part of the question isn't used in the same sense as it is in the second. For an argument to be valid, all terms must be defined consistently from beginning to end.
Invalid, physically they are deceased, although paranormally they may still be alive, and they may "talk" to them paranormally, ergo it cannot be valid.