The old debates over the nature of the soul are best understood through our modern debates over how to define life, because the attempt to define life has the sort of universality and reputed importance that the attempt to define “soul” used to enjoy. For example, when we ask “what is life” it is understood to admit of materialist response, a spiritual one, a religious one, etc. The ability to define life is also reputed to be of extreme significance, and so there are multiple attempts to provide an answer; the disciplines we call religion, philosophy, politics and science all weigh in- as they must.
I’ve read a good handful of the attempts to define life, and almost all of them would be helped out by some basic tenets of the philosophia perennis. One point in particular stands out: in most of the debates and discussions I read, it is assumed that the definition of life must be absolutely univocal for everything that is called living. In other words, it is assumed that the definition must apply in exactly the same way to an Olympic athlete and a dormant cactus. This presupposition will obviously be reductive, for one will be forced to say that our own life is nothing above what one finds in a tree in the snows. Aristotle and St. Thomas both saw life as admitting of more and less perfect instances, that could be grasped only by an analogous concept. This is how we tend to speak of it too: when someone asks “when does life begin” or “what is the meaning of life” the word life means human life. This is what Aristotle would have called the primary analogue, whose definition is found in different ways in different living things.
Many of the definitions also tend to fall into the sort of traps that Aristotle noticed long ago: on the one hand, they will seek to define life dialectically, or wholly on the part of form (power to reproduce or react to stimuli or that which yearns for God), or on the other hand seeking to define wholly on the part of matter (the presence of an enzyme catalyst, a genetic code). This is not to dismiss what they say, because all of these things will be involved in the definition of life, but I still would hold that it is only with Aristotle’s distinctions that one is capable of allowing for a synthesis of the various truths one finds in different disciplines.