The human soul is that which gives life to the human being. For Plato, most Christian theologians of the first millennium AD , Descartes, and many others, the soul was the essential immaterial part of a human, temporarily united with its body. Aquinas also held this, while emphasizing that union with a body was the natural state for a soul.
Most modern philosophers deny the existence of an immaterial soul.
One strong argument for the existence of such a soul given in essence by Descartes is this. I am now conscious. But a thing such as I am can only continue to exist if some part of the thing continues to exist. So I must now already have an essential non-bodily part, i.e. a soul, if my continued existence is to be logically possible. 1. pneuma The Oxford Companion to Philosophy 2.