The soul is an immaterial spirit that many religions claim resides in a human's body during life. It is supposedly responsible for consciousness, and is usually considered to be immortal. In Christianity, one's soul goes to heaven, hell, Purgatory, or simply the spirit world in the case of the Latter Days Saints movement , after death. The belief in a distinct body and soul is referred to as mind-body dualism.
Some anthropologists have suggested that belief in the soul came about as early humans attempted to understand what happened when someone died. One of the first observations that can be made of a dead person is that he is no longer breathing. It appears to be no great coincidence that words for soul or spirit in many languages have etymologies linked to concepts like "air" or "wind."
Some examples of this in other languages:
Ancient Hebrew: rûach רוּח - wind; by resemblance breath, that is, a sensible (or even violent) exhalation (from e-Sword, translated as spirit in Genesis 1:2 KJV)
Swedish: Compare "ande" (spirit) and andlighet (spirituality) with the verb infinitive "att andas" (to breathe)
Interaction between soul and body not explained
If the body and soul are entirely independent, nobody knows how the soul interacts with the body. Since it's immaterial, physical things cannot influence it, yet it somehow communicates with the brain and vice versa.
The idea of a soul raises many questions about relativity. If the soul doesn't occupy our universe, then it shouldn't experience time. Since we do feel time, it cannot be immaterial. Since it occupies our universe, this raises many questions about its composition. Is it made of the basic particles of which all matter is comprised? If it's an entirely different type of particle, what's its mass? Is it influenced by the four fundamental forces? What's its charge? How does it manage to hold your consciousness? None of these questions have ever been answered. Therefore, using the soul to explain something is using a term that isn't defined.
Role of the soul
Just what information does the soul contain? Science has proven that certain parts of our physical brain are used for emotions, other parts are used for memory, logical thinking, and the pacing of one's breath and heartbeat. Damage to a certain part of the brain can drastically affect one's personality, memory, or thought processes. And if the soul contains our memories, then what is the point of our memories being stored in our physical brain as well? Is Alzheimer's or amnesia a problem with the brain or a problem with the soul? If our behavior and personality is explained by physical processes, we can dispense with the idea of a soul by Occam's razor.
The introduction of certain chemical drugs to the brain can drastically change one's thoughts, memories, behavior, emotions, judgment, etc. Are these drugs somehow acting on the soul and changing properties of the soul?
It appears that the word translated "soul" in the Old Testament meant life or breath. This word, nephesh, is used to describe the life of animals. Though in modern Judaism and Christianity animals do not have souls, English translations of the Bible usually translate nephesh differently at different times to fit Christian theology. The Old Testament thus does not support the idea of each human being having a soul that is different from an animal's life to the extent that believers assume, if it supports the idea at all.
Lack of evidence
When a mind is observed, it is always accompanied by a body. Therefore, they probably cannot exist apart. This rules out the traditional concept of a soul.
"But the vulgar prejudice, that body and mind ought always to accompany each other, ought not, one should think, to be entirely neglected; since it is founded on vulgar experience, the only guide which you profess to follow in all these theological inquiries."
There is little consensus between or within religions as to when the soul enters the body. If life begins at conception, presumably the soul enters the body at this time. There is also little agreement as to if the soul is new or if it pre-existed. If non-human animals do not have souls, given the evolutionary origin of humans, it is difficult to specify exactly which ancestor of humans had a soul, when their parents did not.