An Aristotelian-Thomistic Discussion of the Definition of Life

Many deny or doubt that life can be defined. For example, biochemist W. S. Beck affirms: ‘[virologist Norman] Pirie is correct: “life is beyond rigorous definition.”’ ⁽¹⁾ Philosopher of science, Jean Gayon maintains: ‘recognition of “life” has always been and remains primarily an intuitive process …. However, we should not expect, then, to be able to draw a definition from this original experience, because our cognitive apparatus has not been primarily designed for this.’ ⁽²⁾ Biology textbooks typically do not propose a general definition of life, but rather point to certain properties of living things, such as metabolism. ⁽³⁾ When scientists were asked what life is in a survey made in 2000, the most common answers were in terms of self-maintenance, reproduction, and the ability to evolve, sometimes proposed separately and sometimes in combination. ⁽⁴⁾

What I intend to do is to lay out the view of Aristotle and Aquinas. Aristotle defines life as the capacity for self-motion: ‘It is impossible to say that their motion [i.e., of light and heavy bodies] is derived from themselves: this is a characteristic of life and peculiar to living things.’ ⁽⁵⁾ Aquinas, in his commentary on this passage, agrees with Aristotle: ‘to move itself belongs to the notion of life and belongs properly to the animate; for we discern the living from the non-living by motion and sensation, as is said in De Anima, Bk. 1.’ ⁽⁶⁾ It is our shared experience that living things, such as squirrels, see and move themselves from one place to another, whereas non-living things, dead or other, do not. While not all living things move themselves in the obvious ways that squirrels do, they all move themselves in some manner, such as by growing. (Plants move very slowly when they grow. If one is absent for some period, however, it becomes noticeable that they’ve extended themselves to places where they previously were not).

An especially helpful passage in Aquinas concerning life is found in his commentary on the section of On the Soul where Aristotle defines the soul. Aristotle, in determining the soul’s proper subject, sets out three divisions. In the third division he distinguishes the living from the non-living thus: ‘But of natural bodies some have life and some have not; by life we mean the capacity for self-sustenance, growth, and decay.’ ⁽⁷⁾ Aquinas comments:

The third division, which is of natural bodies, is that certain have life and certain do not. Something is said to have life which through itself has nutrition, growth, and diminution. It ought to be known, however, that this explanation is more through the mode of example than through the mode of definition. For something lives not only from having growth and diminution but also from sensing and understanding and being able to exercise other works of life. Whence, in separated substances, life exists due to the fact that they have intellect and will, as is manifest from book eleven of the Metaphysics, although there is no growth and nutrition in them. ⁽⁸⁾ But because in generable and corruptible things, the soul, which is in plants, to which belong nutrition and growth (as is said at the end of the first book), is the principle of life, therefore, here he expounds .having life. by way of example, as it were, as what has nutrition and growth. The proper notion of life, however, arises from the fact that something is apt to move itself, taking motion in the broad sense, according as intellectual operation is also called a certain motion. For we say that things are without life which can only move due to an exterior principle. ⁽⁹⁾

Aristotle cannot be defining life by nutrition and growth for he both attributes life to God ⁽¹⁰⁾ and maintains that God is an immaterial being, ⁽¹¹⁾ one that cannot engage in nutrition or growth. ⁽¹²⁾ This why Aquinas says that in defining life, motion is to be taken ‘in the broad sense,’ one that embraces immaterial life activities as well as bodily ones.

Motion in the strict sense involves imperfection. ⁽¹³⁾ For example, the dog walking to its food dish gets closer and closer to the dish while not being there. And the skin of a person who is tanning gets darker and darker but is not yet fully tanned. Motion in the broad sense includes acts of things insofar as the things are perfect and not lacking, as is the case when they sense and understand. ⁽¹⁴⁾ When one senses, for example, sees yellow, one does not do so gradually, as if partly seeing and partly not seeing, but does so immediately. ⁽¹⁵⁾ There is a motion in the sense of a change from sensing in potency to sensing in act; however, this operation does not become more perfect over time, for example, one does not see the yellow of the banana one is gazing at any better at a later moment than from the first moment one saw it. This goes hand-in-hand with the fact that unlike true motions which all involve some loss (e.g., the skin loses its whiteness in tanning), sensing and understanding involve perfection without any loss. ⁽¹⁶⁾ Ignorance is a privation; it is not something that exists in act that is lost when one senses or learns.

Note, however, that sensing has a relation to true motion. When one opens one’s eyes upon waking in a dimly lit place, one gradually discerns the features of objects, and this is due to a motion in the strict sense, namely, of one’s pupils, which open wider to let in more light. This does not occur, however, when there is adequate light. That there are elements of motion in the strict sense attached to sensing is because sensing requires a physical interaction between what is able to be sensed and the sense organ, and this in turn requires determinate physical conditions. ⁽¹⁷⁾

Aristotle and Aquinas maintain that intellectual understanding more perfectly fits the description of a motion that is the act of the perfect, as it does not involve an organ or any physical change. ⁽¹⁸⁾ The activity of the intellect is only called a motion in a qualified way, insofar as the intellect ‘goes from understanding in potency to understanding act.’ ⁽¹⁹⁾

For our authors, life is an analogous word. Self-motion is not said univocally of plant, animal, and human. ⁽²⁰⁾ The more a living being is in act and the less potential it is, the more alive it is. ⁽²¹⁾ In the case of God, self-motion only applies in an extremely extended sense; all that is left of the notion of motion is act. ⁽²²⁾

It is understandable that scientists would propose a definition of life that would be limited to material things, given the method of science involves observation. As Aquinas notes: ‘Of substances, certain ones are bodies, and certain ones are not bodies. Among substances, the most manifest are bodily substances. For incorporeal substances are remote from the senses and are only investigable by reason.’ ⁽²³⁾ In so short a space, I am unable to present Aristotle and Aquinas’s arguments for the unmoved mover or for the immateriality of the rational soul. Nevertheless, scientists and others who try to define life solely in terms of bodily activities need to be aware they are making a questionable assumption about the nature of life.

In addition to remaining open to the possibility that there exist life forms that do not have nutrition or some other bodily activity, the broad scope of the Aristotelian definition eliminates the concerns of those who think that a definition of life might fail to cover extraterrestrial life, if such was to be discovered. Biologist Imre Friedmann maintains: ‘[A] universal definition of life should help to recognize it when we find it on Mars, or elsewhere in the Universe. But we know only one kind of life, our terrestrial life, and from a single data point cannot validly extrapolate and arrive at a universal definition.’ ⁽²⁴⁾ Would Friedmann deny that lifeless things on Mars or elsewhere are going to be those that can only move due to an exterior principle, while those that are alive will move themselves in some way? It is true that it might be hard to figure out that a given being is performing a life activity. That doesn’t mean that the definition of life is flawed. This is illustrated by the case of viruses. Detailed investigations are required in order to determine whether any of them move themselves to any activity.

Why haven’t scientists adopted Aristotle’s definition? There are a variety of reasons. To name some of them: Scientists often ignore or dismiss Aristotle’s natural philosophy. ⁽²⁵⁾ Some espouse materialism; this closes their minds to seeing the analogous meanings of life. ⁽²⁶⁾ Others reason that the multiple, seemingly gradual steps involved in the origin of life make defining life impossible. ⁽²⁷⁾ Much more could be said here. I will limit myself to considering a potential scientific objection to Aristotle’s definition, as doing so will further clarify the notion of self-motion.

One might object that green plants, when they photosynthesize, are activated by light, and so are not self-movers any more than the non-living streetlamp that turns itself on is a self-mover. It is true that material living things need energy to carry on their life activities. They are not self-movers in an absolute sense. However, they use some of the energy, be it light energy or from the food they eat, to modify themselves, producing new parts of themselves through their internal activity. They are not like streetlamps or other artifacts which do not use the energy powering them to produce new parts of themselves or increase the size of their existent parts. ⁽²⁸⁾

Animals have a greater capacity for self-motion than plants, primarily due to their ability to sense, which ability is often, although not always, accompanied by the ability to move locally. Yet despite this greater capacity for self-motion, there is a reason to regard them more as moved than as self-movers, ⁽²⁹⁾ namely, they are unable to judge their own judgments. ⁽³⁰⁾ A hungry shark that sees a fish cannot reflect on whether or not it should eat it the way humans can reflect on whether they should eat this fish or whether they should eat non-human animals in general. The shark is fixed by nature to desire fish when hungry and to pursue it when there is no obstacle. Whereas we humans can ask ourselves: ‘Should I feel this way? Should I act on this feeling?’ We can control our lives in a way the shark and other non-rational animals cannot, as we can override the desires that we have by exercising free will.

Even rational beings, endowed as they are with free will, are not self-movers without qualification. As Aquinas notes:

Some things are determined in advance [for our intellect] by nature, such as the first principles, concerning which it cannot take a different stance, and the ultimate end, which it cannot not want. Whence, granted, that as to something the intellect may move itself, it is nevertheless necessary that as to some things it is moved by another. ⁽³¹⁾

Aristotle and Aquinas maintain that God alone is not determined in any way by some exterior principle. God is pure act; God’s understanding is not other than himself. ⁽³²⁾ The self-motion in living beings other than God is never independent of some form of being moved by another.

As for AI, Aristotle and Aquinas would maintain that it is not alive on the grounds that it is an artificial thing rather than a natural thing; thus, it does not even have its own tendency to move, much less to move itself. Non-living natural things tend to move in certain ways because of what they are. Iron moves towards a magnet and aluminum does not. Non-living natural things do not move on their own, however; for example, it is the force of the magnet that makes iron move. By contrast, a plant doesn’t grow because the sun pulls it in some manner. Rather, the plant uses light in processes that result in the plant growing towards light. The plant not only tends to move a certain way, as is the case of iron, but it also tends to move itself.

Artificial things, unlike natural things, do not have tendencies that are properly their own. Whatever they tend to do is a function of the tendencies that belong to the natural things that compose them. For example, the fact that a flashlight tends to produce light is only due to our arrangement of natural things whose tendencies we harness and order to this function. We arrange chemicals in the battery that tend to produce electricity. And we add a switch made of a material that tends to conduct electricity, so that the electricity can flow from the battery to the bulb, whose tungsten filament tends to glow when electricity flows through it. The case of AI is no different. Its tendencies to move are the result of the tendencies belonging to the natural things that it is made out of. Since AI has no tendency to move that belongs properly to it, it cannot possibly have life. ⁽³³⁾ The reason some people wonder whether it is alive is because its output resembles that of human intelligence. In fact, however, AI knows nothing. AI is no more intelligent than a book, despite writing the book, so to speak. It merely mimics human intelligence. Like any other artificial thing, AI is lifeless.

I would like to close by contrasting Aquinas’s understanding of Aristotle’s notion of life as I have presented it with that of the contemporary Aristotelians, David Oderberg and Edward Feser. According to Oderberg: ‘life is the natural capacity of an object for self-perfective immanent activity. … Living things, unlike non-living things, exercise immanent causation: this is a kind of causation that begins with the agent and terminates in the agent for the sake of the agent.’ ⁽³⁴⁾ Feser, following Oderberg, holds that nutrition is an immanent activity:

The mark of a living or organic substance is that in addition to transeunt causation it exhibits immanent causation, in which the effect remains within the agent and perfects it. An animal’s digestion of a meal would be an example insofar as it allows the animal to stay alive and grow (though there are also external or transeunt effects like the excretion of waste products). ⁽³⁵⁾

Both thinkers misconstrue the meaning of ‘immanent.’ For Aristotle, an immanent activity is an activity which is perfective of the agent rather than of some product resulting from the agent. ⁽³⁶⁾ Immanent activities are divided against transient activities whose perfection lies in their products and not in the activities themselves.

Aristotle and Aquinas acknowledge that plants and animals, unlike non-living things, take in materials from the outside and through their own activity transform some of these materials into their own substance. The activity of nutrition, however, is not of itself perfective of the agent, but rather of the ‘new flesh’ it produces. This product, the ‘new flesh,’ perfects the organism as a whole. ⁽³⁷⁾ Immanent activities have no product but of themselves perfect the agent. ⁽³⁸⁾ Seeing is an immanent activity, as it perfects the agent rather than perfecting some product internal or external to the agent. Aristotle and Aquinas do not use the immanent–transient distinction when discussing life forms, because it does not suffice to separate plants from non-living natural things, for all the life activities of plants are transient, as are all the activities of non-living natural things. Whereas their own definition of life does allow them to do so. ⁽³⁹⁾

A correct understanding of the nature of plants requires recognizing that they do not have immanent activities in the Aristotelian sense of the word. Plants maintain themselves in existence through their own activities which have an internal product and, in this way, differ from the non-living; however, they do not perform immanent activities – all of their activities have products – and in this way, plants are similar to non-living things, all of which lack immanent activities. Consequently, the Aristotelian distinction between immanent and transient activities is important for an accurate assessment of how plants differ both from the non-living and from animals and humans. ⁽⁴⁰⁾

I hope to have shown that the definition of life advanced by Aristotle and Aquinas is eminently defensible. It applies to all living things – as a good definition must – albeit by analogy, and it fits our common experience of how living things differ from non-living ones.