For the Greeks, the soul is what gives life to the body. Plato thought of it as a thing separate from the body. A human living on earth consists of two parts, soul and body. The soul is the essential part of the human – what makes me me. It is the part to which the mental life of humans pertains – it is the soul which thinks and feels and chooses. Soul and body interact. Bodily states often cause soul states, and soul states often cause bodily states. This view is known as substance dualism. It normally includes the view that the soul is simple, that it does not have parts. If an object has parts, then one of those parts can have properties which another part does not. But for any experience that I have, an auditory or visual sensation or thought, it happens to the whole me. Plato also held that at death, soul and body are separated; the body decays while the soul departs to live another life. Aristotle, by contrast, thought of the soul simply as a ‘form’, that is, as a way of behaving and thinking; a human having a soul just is the human behaving (by moving parts of the body) and thinking in certain characteristic human ways. And just as there cannot be a dance without people dancing, so there cannot be ways of behaving without embodied humans to behave in those ways. Hence, for Aristotle, the soul does not exist without the body.
Christian theology, believing in life after death, found it natural to take over Plato’s conception of the soul. But in the thirteenth century, St Thomas Aquinas sought to develop an Aristotelian conception modified to accommodate Christian doctrine. The soul, Aquinas taught, was indeed a form, but a special kind of form, one which could temporarily exist without the body to which it was naturally fitted. It has always been difficult to articulate this view in a coherent way which makes it distinct from Plato’s. Descartes restated Plato’s view. In more modern times, the view that humans have souls has always been understood as the view that humans have an essential part, separable from the body, as depicted by Plato and Aquinas. The pure Aristotelian view has more normally been expressed as the view that humans do not have souls; humans consist of matter alone, though it may be organized in a very complicated way and have properties that inanimate things do not have. In other words, Aristotelianism is a kind of materialism.
If, however, one thinks of the soul as a thing separable from the body, it could still cease to exist at death, when the body ceases to function. Plato had a number of arguments designed to show that the soul is naturally immortal; in virtue of its own nature, because of what it is, it will continue to exist forever. Later philosophers have developed some of these arguments and produced others. Even if these arguments do not show it (and most philosophers think that they do not), the soul may still be naturally immortal; or it may be immortal because God or some other force keeps it in being forever, either by itself or joined to a new body. If there is an omnipotent God, he could keep it in existence forever; and he might have revealed to us that he is going to do so.