Introduction
Modern Biology textbooks often begin with some variation of this question. The answer includes some assortment of the following characteristics: Made of Cells, Organized (or specialized), Grows and Develops, Reproduces, performs Homeostasis, and performs Metabolism. These lists vary. Some include a couple more attributes, some cut a few, and some group some together. My thesis in this essay is that there is a better principle to determine whether a thing is alive: namely, Aristotle’s notion of living things as self movers. Obviously, this expression will need explanation. I will be drawing on the tradition of Aristotle and St. Thomas Aquinas to do this. I will argue that the modern conception actually fits nicely into the more explanatory and more succinct principle offered by Aristotle and Aquinas.
I will proceed in the following way. First, I will explain St. Thomas’s notion of living things as self-movers. Second, I will critique the modern description of life as an amalgamation of features. Third, I will show how the modern view can be incorporated into the Aristotelian model.
Living Things as Self-Movers
St. Thomas Aquinas tackles this question in I. 18. 1 of the Summa. He begins by outlining this principle of self-movement. The difference between a living animal and a dead one, is that a living one can move itself, while a dead one needs to be moved by another. Next, he describes how movement needs to be taken generally. Movement, for Aquinas, is closer to the word change. He is not limited only to locomotion when he describes movement. Rather, there are several senses of this term. An ice cube that melts into a puddle of water undergoes a kind of motion, or change. Similarly, a student who masters the spanish language has moved from a potential knowledge of spanish to an actual understanding. Movement, then, can mean any kind of change for St. Thomas. Non-living things can move. Rivers can flow, rocks can roll and planets can orbit. The key difference is in what is causing the movement. Non-living things are always moved by something outside it. Living things are agents, and so act on their own. Living things move in and through themselves.
So, a living thing is simply that which can exercise change on or through itself. A squirrel scouring the ground for nuts is executing complicated actions ordered towards specific goals. Similarly, a sunflower moves itself in accord with the sun throughout the day to obtain maximum amounts of sunlight. Trees and Shrubs grow in a way that allows them to obtain sunlight. Even simpler than this though, the squirrel works to regulate an internal environment that is comfortable for the squirrel. Just as a sound engineer modifies all the dials and modules in a studio to optimize sound, the squirrel tinkers with all sorts of internal hormones and materials to be able to function well. The same can be said for plants who change the amounts of chemicals they take in so that water can be best preserved. Are these not types of self movement? The organism is executing and acting on the world to preserve itself. This, I think, is what differentiates the living from the non-living for St. Thomas Aquinas.
Critique of Modern Biological Description of Living Things
As I stated in the introduction, modern biology considers things to be living that check the boxes in a relatively standard checklist. This checklist includes being made of cells, conducting homeostasis, metabolizing energy, growing and developing, responding to stimuli and reproduction. There are small variations in this list, but this is a pretty standard one. I think there are a couple significant problems with looking at the question of living things in this way.
First, lists are not definitions. If you asked me what a human being was, and I proceeded to list off human beings, I failed to understand the question. Even if I listed off attributes that humans typically have, this is still not a helpful definition. To say that a human being is something that has arms, larynxes, and medicine cabinets does not answer the question.
Second, definitions need to give a principle that sets the limits between types of things. Taking the human being example, we need to describe what makes them in common with other things, then appeal to what differentiates them. Animality is what it has in common, rationality is what separates them from the animals. A human being, then, is a rational animal. This definition gives the principle by which you can include and exclude it against other things. Even a helpful list of attributes (hairy, bipedal, joke-telling, etc), doesn’t accomplish this.
The definition of life needs to be able to give the principle that separates the living from non-living, while also telling you something about all living things. I think that Aquinas’s definition is actually quite good here. Living things are substances (things that contain a unity), that are capable of self-movement. Rocks are substances, but they don’t have the ability to move according to their own powers. Squirrels are substances who also have the capability for acting on the world around them. So long as we adequately explain what we mean by the term self-mover, this definition seems to be in good shape.
Reconciliation of Modern and Ancient Perspectives
While the modern perspective misses the mark from being useful as a definition, I think it can be reconciled into the Thomistic perspective. I think that most of the things included in the checklist are actually examples of being a self-mover. Let’s take each one individually.
Reproduces. In Biology, reproduction involves a passing down of genetic material between generations. This looks different depending on the type of organism we are talking about. Regardless though, this is an act that the organism carries out. And so, is an act of self-movement.
Responds to Stimulus. This characteristic means that living things react to changes in their environment. Living things are able to act differently depending on their external surroundings. Again, this presupposes an ability to act and change on its own.
Metabolizes Energy. Living things convert between food and energy. Plants take in energy and convert it to food (while using some of it). Animals take in food and convert it into energy. All living things though have this relationship between food and energy. Again, this is an example of self-movement. Being able to convert chemicals is something that requires agency in some capacity.
Uses Homeostasis. This refers to living things regulating their internal conditions. When we get cold, we shiver. When we get thirsty, we seek water. Lizards seek shade to avoid overheating. Some viruses will hunker down into a seed form if they sense the presence of certain immunological responses. This is another great example of self-movement, or agency.
Grows and Develops. This characteristic is actually two distinct features. Growth refers to change in size. This occurs in part due to mitosis. By increasing the number of cells an organism has, it grows in size. Development refers to the different stages of life an organism goes through. Humans go from fetus to toddler to teenager, etc. Both of these also require the organism to be orchestrating and enacting these changes on its own. Rivers grow because of external factors, but foxes grow because of internal movement.
Made of Cells. This doesn’t factor into this definition much, but is a can of worms that I intend to write about soon. Modern biology considers living things to be nothing but the cells that make them up. Aristotle and Aquinas would recognize that this ignoring of formal causation is actually a root to this problem. Living things have a formal cause that makes them alive. This formal cause is the soul. Corpses still have a material constitution of cells, but they lack the unifying principle of a soul to make them into the unified organism it used to be. I think this is a large reason why modern science can’t see what the principle that determines life is. By ignoring formal causation, science loses its ability to see underlying principles. In this case, agency or self-movement.
Conclusion
I think the modern list of characteristics of living things fails as a definition. It does not point to a principle that separates living things from nonliving things. It seems to me that Saint Thomas Aquinas’s account of living things as self-movers does allow us to classify things according to a real principle. Nonliving things can only be acted upon. They are passive recipients of what occurs in nature. Living things, though, are actors upon the world. Living things do things. While the characteristics are nice examples of this, this is ultimately secondary to having a principle in place. By returning to this principle, science has a firmer leg to stand on in building up a true study of living things.
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