What if the soul was made up of two parts, I will call them the Id and the Ego. The Id is the life force of the soul, it is what powers the body and is molded by spell casters into spells. The Ego is the personality of the soul, this is were memories are held, this is also the seat of the mental attributes.
When a new creature is created the Id is taken from the plane of positive energy and powers the new creation. The Ego is a blank slate ready to be developed over the life of the creature. As the creature lives it's life the ego grows with experience and with the development of its mental powers. The Id grows in proportion to the ego, if both become strong enough the creature can wield the power of it's id to cast spells.
When the creature dies the id is returned back to the plane of positive energy and the Ego goes off in the river of souls to the after life. Especially strong Egos will leave a mark on the Ids that they held, these marks will manifest themselves as traits in the creatures that that id calls home in the future. These traits will shape the developing Ego, also the type of creatures favored by marked ids. Some Egos are so strong that they effect those round them, sometimes for generations.
Sorcerers are creatures who descended from those who were either marked by those of incredibility strong egos, or who themselves were marked. This mark increased their id to the point were they are almost creatures of pure magic, but because they themselves did not work to get this power they are in a constant power struggle to control their massive id. Because of this they tend to invest their time into developing an extremely overpowering personality to try to control their power. Their life is spent trying to control their id by developing their ego, but by developing their ego they enlarge their id.
Wizards are creatures who through consistent study have developed their intellect to incredible heights. This powerful intellect is then used to mold their id into powerful spells. For them id and ego are limited resources that must be managed at all times. When ever a Wizard memorizes a spell they are not memorizing words, but rather they are molding id into a usable form. Once the spell is cast that id that was prepared is used up and the spell can not be cast until more id has been molded.
Clerics are creatures who devote their lives to deities, and through this devotion they gain power. As the cleric grows in their devotion they are increasing their ego by gaining wisdom. With this their id become large enough to be molded like a wizards, but clerics lack the intellect to mold it themselves. So they pray to their god to receive their power, or rather to have their id molded into a usable form. Clerics can only gain power equal to their own id for they are not receiving power from their god, only having what they already have shaped by their god.
