People often use “soul” and “spirit” interchangeably. That can get confusing, because the two terms denote concepts that differ in subtle but important ways, especially in writers of the ancient world.
It helped me tremendously to learn (from Aristotle) that the soul of an organism – plant, animal, human – is its form. When that form is united to matter – when matter assumes that form and concretely expresses the values that it specifies (as height, weight, rationality, homeostasy, volition, motility, consciousness, and so forth) – the union is an integrity, a single being with both formal and material character: a body ensouled, a soul embodied. It then lives.
The life of an organism is its spirit. This is clear from the etymology of “spirit,” from the Latin spirare, “to breathe.”
Forms in and of themselves have no concrete actuality. Rather than themselves real, they are ways of being real. So the soul by itself is not a subject of experience. Nor by the same token is the body by itself a subject of experience. But a body ensouled is alive, and can be a subject of experience.
Not all sorts of souls need to be embodied in order to live. There are other sorts of lives than embodied lives. Angels, for example, are not embodied. Nevertheless they live – not in the sense that they eat, drink, breathe, and so forth, but in the sense that they can act and are subjects of experience (although not of the sensations mediated by bodies (sensation is not the only medium of experience)). It is not uncommon to hear angels described as “purely spiritual” beings, but this expression is not quite accurate: all spirits are spiritual, it’s just that some are embodied, some not. Angels then should properly be described as immaterial.
I grant of course that this distinction can get muddled even in Greek and Latin. Pneuma and spiritus can both be translated pretty reliably as spirit; but psyche and anima can be translated either as soul or as spirit, depending upon the context. I find nevertheless that, when I bear clearly in mind the distinction between soul and spirit that I have here set forth, it is easy to tell when an ancient author is using anima or psyche to mean the soul properly so called, and when to mean a soul concretely implemented in a life: a spirit.