Soul : The part of human beings that thinks and feels, and is immortal.
As defined, this is mostly a religious hypothesis, because there is no evidence for survival after death (which is indeed a contradictio in adjecto). The scientific hypothesis here is that the brain is the organ with which one thinks and feels, and that one's experiences, thoughts and feelings cease when the brain ceases to live.
Even so, there are at least two uses to which the hypothesis of a soul may be put, both of which are much tied up with religion.
1. Existence after death: In part, and apart from supposed immortality, a soul is a kind of self or I, which seems to be at least a necessary assumption for human beings to cooperate, since no one can feel another's feelings or think another's thoughts, while one needs to come to agreements, share ends and practices, and live to some extent in the same commonsensical world.
Thus, the soul may be seen as the self after the body has died, and indeed in this guise a soul may be supposed to transmigrate to a new body after the death of a body it inhabited and guided. This is a rather common religious hypothesis, but there is little evidence for it, since all children seem to start life completely ignorant of the world they have been born in.
Human beings normally, and apart from great personal problems, take themselves to want to live and survive, and thus the hypothesis that one has a soul that even survives the death of one's body may be seen as a rather obvious piece of wishful thinking, as is the heaven believers in souls like themselves to end up in.
2. Judgment after death: When the soul persists after the death of the body it animated, there is the logical possibility it may appear before the maker(s) of the universe, and be judged for its commissions and ommissions while living on earth.
This is also a common religious hypothesis for which there is no evidence. It must have frightened millions, and also must have supported millions with the hope that in heaven they would be rewarded for leading blameless if unpleasant lives on earth and finally also everything would be explained, including the many cases of evident injustice, immorality, cruelty, murder and persecution all men know of, that are not so easily explained rationally by a human intellect on the basis of the hypothesis that there is an infinitely good, benevolent and omniscient god.
It is probable that this religious belief has been quite effective morally, in that the belief that one would be judged after one's death may have kept many from doing evil, and in that priests and clergy have widely used one's supposed divine judgment when dead as a way to influence the faithful and threaten the faithless. Also, while many may have refrained from harming others out of fear of divine judgment, there have also been many who murdered what they held to be the unfaithful in the belief of being themselves rewarded for such killings.
If we consider the question of what is the evidence for the existence of an immortal soul, the answer must come in two parts.
First, at present there is no good explanation at all of how the brain produces one's experiences, thoughts, feelings, beliefs, desires, values and ends, although there is very good evidence that it is the brain that produces these, or at least is necessary for their production.
One important reason for this fact is that the brain is the most complex organ one has, and so far it was difficult or impossible to study living brains, though this last restriction is rapidly disappearing through things like PET-scanners and nano-technology.
Second, there is hardly any evidence evidence for the existence of an immortal soul: There are no ghosts, no messages from the death, and prayer has been statistically proved to be ineffective. Also, every time the purported existence of gods or messages from the death has been investigated, it has been found to be fraudulent, a hoax, or (self-)deception. (See e.g. C.E.M. Hansel: "ESP - a scientific explanation".)
Hence, the assumption in this Philosophical Dictonary is that there are no souls, and the belief that there are is due to wishful thinking or religious influences in one's childhood.