In my research on cosmology, considering the nature of gods, planes, and the undead, it has become clear why only humans are able to become clerics, why only humans have psionic powers, and why humans are the most powerful magic users. In short, only humankind has souls. While the demi-human races seem to be closely related to humans based on outward appearance, there are profound differences beneath the surface.
The demi-humans are creatures of the natural world, we humans are the interlopers. They are animated solely with the magical energy of the material world, we are animated with a spirit energy from the astral plane. When they die, their material spirit expires with their material body, there is no question of survival after death. For us, death of the body is of the material body only. Our spiritual essence lives on, joining the ranks of the blessed ancestors (or the hungry ghosts).
Relation to the Undead
It is no accident that zombies, ghouls, and vampires are all former humans, and all ghosts have a human visage. Because the demi-humans lack this spiritual essence, they are completely unfamiliar with ghostly hauntings, nor are their bodies susceptible to reanimation. In other words, they are free of the curse of the undead. That particular phenomenon is humanity's burden to bear, exclusively.
Relation to the Divine
However great the curse of the undead, the blessing of the gods is a countervailing benefit. Just as wicked human souls may haunt the living as dangerous hungry ghosts, the blessed souls of the benevolent may assist the living as guardian angels after they pass on from their earthly life. The presence of these helpful spirits is based on the survival of the human soul, and thus, again, a feature of humankind only, demi-humans report no such phenomenon among their own people.
The Astral Mechanism of Psionics
Psionic powers are also based to the spiritual nature of humankind. The basis of all psionic power is the ability to use one's soul to interact with and manipulate the astral realm.
Psionic mental attacks, for example, are based on the hostile focusing of astral energy upon the astral body of another. When you observe a psionic battle while in an ethereal state upon the astral plane, the effects are spectacular. Beams, waves, and particles of various shining colors, are hurled back and forth between astral combatants, either to be deflected aside by a successful defense, or to tear the victim's astral body to shreds.
Psionic domination of another is like a ghost possession, as the astral body of the attacker overwhelms the astral body of the victim, shoving it aside to achieve control of the victim's brain apparatus.
Psionics become adept at projecting their souls, out of the shells of their physical bodies, into the astral realm. Some do this to examine the astral realm for psychic residue of previous events. Some do this to examine another's astral aura for telltale signs of evil intention, as our astral bodies betray our strong emotions.
Some psionics project their souls to spy, as an invisible ghost, seeing and hearing the events of the material world in an ethereal state. Many a supposedly secret conference has been infiltrated in such a wise, to the ruin of men and whole nations! Foolish is the king who fails to take precautions against such a ruse, I can assure you.
Humans, having souls anchored in the astral realm, have the potential to do these psionic feats; demi-humans, lacking such an astral soul, do not. It is both a curse and a blessing, for demihumans are also, as a consequence of their unsoulment, unaffected by psionic attacks.
The faerie magic of the Elves, and the mysterious illusion-making of the Gnomes, are a natural expression of who they are as magical creatures. The science of the human magic user is foreign to them. It appears that the origins of human magic lie in a theft from those races. Perhaps it was a gift, we will never be sure of its original provenance, but what is undeniable is that many of our magical power words and symbols are taken straight from the native languages of the Elves and Gnomes.
However, as their magic is a natural expression of their magical nature, demi-humans are also limited by the scope of that nature. Human magic users, approaching it from the outside as investigators and plunderers, face no such limitation. The human magical arts, although clearly based on the natural demi-human magical abilities, have greatly surpassed them in scope and power. Human magic draws from the magic of the whole world, and includes power words appropriated from other sentient sources, such as giants, dragons, and pixies. As a result, the most powerful magicians in the known world are invariably of the human stock.
Comparative Lifespans Considered
The remarkable longevity of the demi-human races also seems to be connected to their unsoulment. As purely natural creatures, they are sustained by the natural magic of the world. Its mana infuses them, providing them with their spirit of life. As such, their natural lives are not subject to same quick decay that we humans must bear. The life cycle of the demi-human is a languid expression of natural unity with the magical universe.
This helps account for the lack of grasping ambition in the demi-human races, a trait which is all-too-obvious in human society. Everywhere we look, we see demi-humans, as intelligent and creative as humans in all respects, nevertheless living in harmony with their surroundings, and with each other. Gazing at humankind we see a different spectacle, best characterized as a perpetual war of all against all: conquering territory, enslaving enemies, building empires, tearing empires down, with never-flagging energy.
The natural wisdom and long lives of the demi-humans seem to inoculate them against such diseased fanaticism. We act like harried animals, trapped as we are, as spirit creatures surrounded by the material, as mundane creatures surrounded by the magical. They are creatures solely of the magical material world, and this provides them with a peace which humans do not seem to know.