The ancient Egyptians believed that the soul of the deceased was represented by as many as seven spiritual selves, the most important being the ba, the ka, and the akh.  The ka was the life force that had to stay within the body.  It is the part of the soul that partook of the offerings left in a mummy's tomb.  The body had to be preserved to act as the vessel for the ka.  In many cases, a ka statue was created to provide a surrogate vessel for the ka in case the body was damaged.  The ba was the personality, the animation of the deceased.  It was represented by a human-headed bird.  The ba could leave the body and travel to the afterlife during the day, but it had to return at night.  This is one of the reasons that it was important that the mummy's mask looked like the deceased (albeit an idealized representation) so the ba could return to the correct body.  The akh was the "complete" spirit that resulted from the eventual uniting of the ba and ka.