The word "soul" is used by almost everyone with the negative understanding that, unlike physical and material bodies, it represents something that is not physical and not material. But that understanding of soul, in itself and in relation to body, leaves many philosophical questions unanswered.
This most important issue in the understanding of soul occurred early in the history of philosophy. It is the difference between the views of Plato and Aristotle.
For Plato, the soul was a spiritual substance conjoined with a material or physical body. In addition, for Plato that union of soul and body occurs only in human beings. Plants, animals, and other living organisms do not have souls.
In the Platonic view, the immortality of the human soul is self-evident, or at least easily demonstrated, because when death occurs, the soul, having a simple substance, is released and continues in existence.
Wordsworth's "Ode on the intimations of immortality" speaks of the soul as coming from heaven which is its home; and refers to the body as "the prison house of the soul."
The Platonic doctrine of body and soul takes on another form in modern times with Descartes' distinction between res extensa and res cogitans. As for Plato so for Descartes, the human being is an almost inexplicable union of two separate substance -- body and mind. Since Descartes, the mind-body problem has obsessed modern philosophy. As stated by Descartes, that problem is insoluble.
For Aristotle, the word "soul" names the form to be found in the substance of all living matter. A living organism, as opposed to an inanimate substance, is "besouled" -- which is to say "alive." These two words are interchangeable.
For Aristotle, the question of the immortality of the human soul is not raised, even though Aristotle declares that the intellect, which is one of the soul's specific powers, is immortal because it is immaterial. However when Christian theologians consider the immortality of the human soul, they must also affirm the resurrection of the body, because the imagination is a necessary but not a sufficient condition for the exercise of human conceptual thought.
From the purely philosophical as opposed to the theological point of view, the most that can be said is that the immortality of the human soul is possible, but its actuality cannot be philosophically demonstrated.
Adler's Philosophical Dictionary (1995)
Great Ideas from the Great Books (1963)
by Mortimer J. Adler