Of the three you mentioned, Tar is actually the least questionable for me. Ghost is not based on the Iden, but on "ectoplasma", in the ghostly-material sense. There's some explanation further up on this page I believe. Virus is sort of based on Makuta viruses, but I don't think it's as powerful. Tar, on the other hand, is a substance, just like stone or air, and works a lot better as an element than, say, psionics (sorry, pet peeve ).