The soul, according to many religious and philosophical traditions, is the self-aware essence unique to a particular living being. In these traditions the soul is thought to incorporate the inner essence of each living being, and to be the true basis for sapience, rather than the brain or any other material or natural part of the biological organism. Some religions and philosophies on the other hand believe in the soul having a material component, and some have even tried to establish the weight of the soul. Souls are usually considered to be immortal. Many beliefs hold they exist prior to incarnation. 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 may happen to the soul after the death of the body. It also shares as a Proto-Indo-European language root of spirit. The consensus among neuroscientists and biologists is that the mind, or consciousness, is the operation of the brain. They often fuse the terms mind and brain together as "mind/brain".[19] or bodymind. Science and medicine seek naturalistic accounts of the observable natural world. This stance is known as methodological naturalism[3] Much of the scientific study relating to the soul has been involved in investigating the soul as a human belief or as concept that shapes cognition and understanding of the world (see Memetics), rather than as an entity in and of itself.