Well, the formula demonstrates that without mass there can be no energy. This is why there are no ghosts, spirits or spooks. It is easy to understand if you just know what you are viewing. E=mc2
Just because something doesn't have an apparent mass (can go through walls, etc.) doesn't mean it can't exist. Neutrinos have a mass so small that it has not been accurately measured. They have no charge, and pretty much fly through everything without affecting it, given that 6.5x10^10 neutrinos from the sun pass through each square centimeter on earth every second. However, I still don't believe in ghosts.
The only correct answer is that any perceived 'spirit' of a person is an utterly physical phenomenon, mostly composed of neuronal connections, synaptic firing, and hormonal fluctuations. These things cannot be separated from the physical form of a living organism because they are themselves physical.