The soul, according to many religious and philosophical traditions, is a self aware ethereal substance particular to a unique living being. In these traditions the soul is thought to incorporate the inner essence in each living being, and to be the true basis for sentience. In distinction to spirit which may or may not be eternal, souls are usually (but not always as explained below) considered to be immortal and to pre-exist their incarnation in flesh. The concept of the soul has strong links with notions of an afterlife, but opinions may vary wildly, even within a given religion, as to what happens to the soul after the death of the body. Many within these religions and philosophies see the soul as immaterial, while others consider it to possibly have a material component, and some have tried to establish the mass (weight) of the soul.