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. It is believed in many cultures and religions that the soul is the unification of one's sense of identity. Souls are usually considered to be immortal and to 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. Some religions don't even believe in the soul.
Science and the soul
Science and medicine seek naturalistic accounts of the observable natural world. 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.
When modern scientists speak of the soul outside of this cultural and psychological context, it is generally as a poetic synonym for mind. Francis Crick's book The Astonishing Hypothesis, for example, has the subtitle, "The scientific search for the soul". Crick held the position that one can learn everything knowable about the human soul by studying the workings of the human brain. Depending on one's belief regarding the relationship between the soul and the mind, then, the findings of neuroscience may be relevant to one's understanding of the soul.
An oft-encountered analogy is that the brain is to the mind as computer hardware is to computer software. The idea of the mind as software has led some scientists to use the word "soul" to emphasize their belief that the human mind has powers beyond or at least qualitatively different from what artificial software can do. Roger Penrose expounds this position in The Emperor's New Mind. He posits that the mind is in fact not like a computer as generally understood, but rather a quantum computer, that can do things impossible on a classical computer, such as decide the halting problem. Some have located the soul in this possible difference between the mind and a classical computer.
Attempted demonstrations of the soul as distinct from the mind
During the late 19th and first half 20th century, researchers attempted to weigh people who were known to be dying, and record their weight accurately at the time of death. As an example, Dr. Duncan MacDougall, in the early 1900s, sought to measure the weight purportedly lost by a human body when the soul departed it upon death. MacDougall weighed dying patients in an attempt to prove that the soul was material and measurable. These experiments are widely considered to have had little, if any, scientific merit. Despite the fact that MacDougall's results varied considerably around 21 grams, for some people this figure has become synonymous as the measure of a soul's weight. Experiments such as MacDougall's have not been repeated with current precision equipment and research tools, and snopes.com concludes of one researcher that:
- "MacDougall's results were flawed because the methodology used to harvest them was suspect, the sample size far too small, and the ability to measure changes in weight imprecise. For this reason, credence should not be given to the idea his experiments proved something, let alone that they measured the weight of the soul as 21 grams. His postulations on this topic are a curiosity, but nothing more."
Research on the concept of the soul
In his book Consilience, E. O. Wilson took note that sociology has identified belief in a soul as one of the universal human cultural elements. Wilson suggested that biologists need to investigate how human genes predispose people to believe in a soul.