As part of my research, I’m tracing the history of definitions of life in biology and theology. At the moment, my attention is on the different uses of “soul” through time. For the Ancient Greeks, soul could mean many things from the afterimage of a person, preserved in the underworld to a generic principle of life. Some applied it only to humans, but others saw souls in animals, plants, even magnets. Plato’s dialogues provide two interesting perspectives (possibly innovations) for how we look at souls. In the Phaedo, he speaks of human souls as immortal and immaterial, laying the groundwork for the supernatural souls so popular in Christianity. In the Republic and the Timaeus, he speaks of layered souls, with different aspects responsible for appetite, spirit, and intellect. Aristotle builds on this latter perspective and uses it as a systematic description of life.
The Hard Problems of Life
Aristotle thought there were five properties of life that defied easy explanation: nutrition, reproduction, motion, sensation, and reason. These properties could not easily fit into his physical descriptions of the world and suggested something interesting going on among living things, something that required additional insights. They remain problematic today.
Nutrition involves eating or taking nutrients. In short, nutrition is the ability to take that which is not you and turn it into you. Aristotle grouped nutrition and growth together into an ability to repurpose the matter, to make it express your form and substance. In modern terms we would call this metabolism, the chemical reactions present in living organisms.
Reproduction has to do with making copies of your self, duplicating your substance.
Motion for Aristotle has to do with manipulating your environment, changing the world around you. It’s unclear to me that any modern category captures his idea. For him, interactions always had an actor (agent) that brings about a change and a subject (patient) that is changed. Some objects move against their nature (physis, the origin of physics) as when a heavy rock moves upward. An actor is required for this to occur, for example a woman throws the rock. The rock cannot move against its nature (to fall downward) unless acted on by an outside force.
Sensation works much like nutrition, but forms are incorporated rather than matter. Like motion, the idea is foreign to modern perspectives. Aristotle thought that we perceived the world when the form of something entered our sensory apparatus. Matter within our eyes actually took on the forms of the things seen so that those forms could be comprehended. He thought we were literally “informed” when we saw things, just as our form becomes enfleshed when we eat and reproduce.
Reason should be the most familiar concept as it relates to our ability to think about the world. The processes of writing and reading this blog demonstrate reason. Aristotle associated reason with humans – though perhaps not exclusively with humans – and felt that our highest purpose was to reason well.
Animal, Vegetable, Mineral
In trying to construct a systematic science of life, Aristotle divided the world into four categories according to the hard problems, according the abilities associated with life.
Minerals have none of the abilities of living things.
Vegetables have nutrition and reproduction. Note that in modern biology this would include all living things, not just plants.
Animals have the abilities of vegetables, but also possess motion and sensation. They can sense and manipulate their environment. They have preferences and act on them. This is not a good category for modern biologists, because all organisms sense and respond to their environment.
[The kingdom Animalia was proposed by Linnaeus (1707-1778), the father of modern taxonomy, to include organisms that can move; plants were stationary. In the 21st century, we classify things by phylogeny, their historical relationships. The kingdoms Animalia and Plantae now refer to specific lineages of multicellular organisms. Some animals are stationary (e.g., sponges) and some plants move (e.g., Venus flytraps). All organisms interact with their environment, responding chemically and making changes.]
Humans are rational animals. They have all the abilities of vegetables and animals, but can also reason.
Living things, for Aristotle, have their own nature (physis) that relates to their highest ability. Thus, vegetables are fulfilled by eating and reproducing. Animals are fulfilled by moving and acting. Humans are fulfilled by reasoning well. Humans also eat and move and should pursue these goals, but only in subordination to the higher function of reason. Interestingly, Aristotle blurs the categories. He speaks occasionally as though there were a continuum.
In the next post, we will move to Aristotle’s explanation of the abilities and levels in terms of souls.