Souls are the sum accumulation of all physic that departs from a body under certain circumstances. It contains all of the memories and knowledge of an individual, and may be reabsorbed by the original body, or by that of another.
Souls take the appearance of small blue glowing orbs, akin to a plasma globes or a star. The central orb, called the nucleus, has asurface that boils violently and shoots off tendrils, called memory flares, in random directions.
Souls outside of a body decay at a rate of about 1 day's worth of life and memories per second in small flares. The first memories that shed are the most recent, but as decay progresses the age of the shed memories becomes more random. As the final memories are shed, the soul explodes in a flash of light.
The decay rate of souls is fairly well understood to be constant under normal conditions, but it may be slowed down under certain conditions. Small stoneware jars appear to work best, reducing the decay rate by up to a factor of ten.