What I think Aristotle's "Nutritive Soul" Means For Biological Cells

Aristotle was one of the early thinkers that developed ideas about the Soul, but his ideas about the Soul are quite different than what one might expect. Many people know of Aristotle as a philosopher, but a lot of people don't realize how much time he spent on studying biology. In fact, the period of his life where he didn't publish a lot of philosophical writings was when he is known to have spent a lot of his time studying animals [1].

I recently read Aristotle's On the Soul, because I was initially curious how the concept of the Soul had developed. But as I read the book, I realized that Aristotle went about the question of the Soul in an incredibly scientifically minded way. As a book, I thought it was a great example of how scientific study can inform one's understanding of concepts that are more philosophical in nature, such as on the subject of the Soul.

We usually think that there is something rather special about living things that seem to distinguish us from the more simple inanimate objects like rocks. This always leads many to think that we are also made of something beyond typical material substances (simple collection of atoms), which for the lack of a better term we may identify as our souls. As I read Aristotle's On the Soul, I started to realized that Aristotle is trying to explain something really deep about the nature of biological life itself.

A quick disclaimer, I am not a specialist in philosophy and am more of a philosophy enthusiast. I also haven't read a lot of Aristotle's other works, which I am definitely looking forward to do more in the future. I have some training in science, but I am not a professional in any area of science. Please read this article as my own (developing) view on Aristotle's work.

Aristotle goes over many different topics regarding the soul in his book. For Aristotle, all living things, plants, animals and humans have souls, but they each have different kinds of souls. The soul that has to do with biological life in general is the nutritive soul which plants have. According to Aristotle, there are also the sensitive and appetitive souls, which we share with animals, and on top of this we humans also have the rational soul, which let's us have rational thoughts. I'll be focusing on the nutritive soul in this article, particularly zooming in on what the soul is for individual cells, since cells are considered to be the smallest lifeforms. I'll format this article in terms of the questions I had and the answers that Aristotle gave in his book. The first question I had when I started reading the book is

1. What exactly did Aristotle think the soul is? (and specifically the nutritive soul)

And the second point that I'll be focusing on is

2. In light of what we know about biological cells, what would Aristotle say if he was asked what the Soul of a cell is?

Aristotle's conception of the Nutritive Soul

Aristotle begins to define what he means by the term 'soul' in the beginning of Book II in his book On the Soul. The book is about 100 pages long, and he splits the whole book into three smaller parts, and he refers to each of them as Book I, II and III. So in Book II, he points out that there are both living and non-living bodies, and that the body of an animal or human cannot be said to be a soul. A 'body' to Aristotle means just the plain (static) matter that makes up a thing. He then uses the term form to describe the soul as the form of a natural body having life potentially within it. Simply put, the form of a thing is the definition, or essence of the thing [2]. He then gives the definition of a soul that, according to Aristotle, is applicable to all kinds of souls,

[the souls is] the first grade of actuality of a natural organized body.

Here he means actuality in that a collection of matter on its own is 'pure potentiality', i.e. it can potentially take on any kind of form or structure. An example of having 'pure potentiality' for a non-living thing is a slab of marble, which can only receive a form when it is molded into some kind of statue [1]. I'll discuss this particular definition of the soul of his in more detail below. He also says that the soul is

the definitive formula of a thing's essence. This means that it is 'the essential what-ness' of a body. [II, 1]

Aristotle goes on to give an illustration of what this means by using the example of an eye [II,1]. According to Aristotle, if an eye could exist on itself, then the act of 'sight' would have been what we call the soul of the eye, since the essence of being an eye is to 'see'. However, once an eye looses it's ability to see, it has then lost the 'what-ness' of what it means to be an eye, and now even though the eye is roughly the same shape and structurally the same as when it was 'seeing', it has nevertheless lost its essence; its soul.

For Aristotle, since the soul is nothing but the actuality of a certain body, it is not meaningful to ask whether a soul and body are united, because by definition the soul is what gives life to the matter that composes the body [II,1].

He then describes what it means for a thing to be 'living'; the ability to think, perceive, to originate movement or rest, self-nutrition, decay and growth [II, 2]. Out of the different powers a living being can be capable of, he mentions that the ability of self-nutrition is the most fundamental, since every living being from plants to humans have the capacity to feed themselves, while the other abilities like sensing, according to Aristotle (with his understanding of plants at the time) is only possessed by animals [II,2].

At this point of reading the book, I was really struct by how real the soul is to Aristotle. It is almost unquestionable that the soul exists when you look at it from this angle, where the soul is defined as the essence of the living object. In another place, Aristotle says that 'the soul is the cause or source of the living body [II, 4]'. He understands matter on its own to be motionless mass, and thus he attributes its ability to come alive to some source, which he calls the soul. As he says in the opening of the book "the study of the soul must fall within the science of Nature [I, 1]", in so far as the soul relates to describing what is seen in the body. Although, it is important to point out that Aristotle doesn't think that the study of the soul falls entirely in the realm of science [1]. Of course his 'science of Nature' was not yet the scientific understanding of nature that we have now, but it is still clear that his study of the soul will be grounded on his observations of the natural world.

I particularly like his definition I quoted above where he states that the soul is the "actuality of a natural organized body." The term 'organized' is very important here I think. We now know how important the organizational structure of the cell is for cells to perform activities that we associate with life [3, 4]. In fact, the ability for a myriad of mechanisms to be organized in a particular hierarchy is seen as a big distinguisher between living and nonliving entities [3]. Just take the nucleus of the cell for example. The nucleus produces ribosomes, which are large biomolecules, that are involved in the synthesis of proteins, and proteins are in turn involved in a huge number of processes in the cell [6]. There are about 700 different protein species in the nucleus of our cells alone [4]. The nucleus itself is organized into spatially distinct subsections that are specialized in performing different tasks that result in the production of ribosomes [4]. These spatially nested liquidy regions in the nucleus are occupied by different types of proteins that are involved in the production of ribosomes, displaying the spatial organization of proteins within the nucleus [4]. This is just one example of how cells are able to hierarchically organize their material to perform different biological functions in the cell [4]. So Aristotle's emphasis on highly organized collection of matter appears to be pretty aligned with what we now know about the complex nanoscale organization required for living things to come alive at the microscopic level.

But with regards particularly to this definition ([the souls is] the first grade of actuality of a natural organized body), I kept wondering what 'actuality' would mean in the particular context of the soul of a living cell. Aristotle explains how there are different levels or powers of the soul, each characterized by the different powers of self-nutrition, sensation, thinking and motivity. As I pointed out above, the power of self-nutrition is shared by all 'living' things and thus is the most primitive type of soul according to Aristotle [II,4].

Since we now know that cells are the basic unit of life, and that they have the capacity for self-nutrition, explaining what exactly is actualizing a cell would be fundamental in understanding what the word 'actuality' would mean with regards to our current knowledge of biology. As he says in the section where he talks about the nutritive soul, 'the soul is the cause or source of the living body [II,4]', thus he is really using the word 'actuality' in this sense of 'causing' life. As explained in the Stanford Encyclopedia of Philosophy article by Christopher Shields, Aristotle is applying his theory of causation to say that the soul is the formal cause of the cell, being the form, or essence, that gives it life. Aristotle's theory of causation gives four different kinds of causes: the material cause, the formal cause, the efficient cause and the final cause [1]. In the case of the cell, we can say that the material cause is obviously the body of the cell (the proteins, lipids, water etc.), the formal cause would be the soul that gives the body life. The efficient cause is the reason for the cell to exist, so for Aristotle I believe he would name the efficient cause to be the parent cell, since for an animal he says that its parents are the efficient cause of that animal [1].

But why actualize?: The purpose of the Nutritive Soul

But I still had the question of why would such a thing as the 'soul' want to actualize or cause life? So in Aristotle's terminology, what is the final cause of the cell to perform self-nutrition at all? I didn't really catch this upon the first time I read the book, but as I read over this section where he talks about the nutritive soul, I found that Aristotle says that we can 'call things by the end they realize [II,4]' and that

the end of this soul [nutritive soul] is to generate another being like that in which it is, [thus] the first soul ought to be named the reproductive soul [II,4].

So even though the most primitive soul is the nutritive soul, if we were to refer to this soul in terms of its end that it wants to actualize, it is to reproduce another like itself. But then I also wondered, thinking in the context of the cell, why would a cell want to generate a being like itself in the first place? Aristotle's answer seems to be given in the following

Since then no living thing is able to partake in what is eternal and divine by uninterrupted continuance (for nothing perishable can for ever remain one and the same), it tries to achieve that end in the only way possible to it, and success is possible in varying degrees; so it remain not indeed as the self-same individual but continues its existence in something like itself ... [II,4]

So for Aristotle, the soul's final goal is to partake in what is eternal. This seems to be the key reason why Aristotle's view of life does not fall into the category of the reductionist that sees life to be fully explainable by mechanical and material causes. As Shields points out in his article, the way Aristotle speaks of the soul causing life, prevents one from defining and reducing life to simply the capacity for taking on nutrition [1]. Thus, Shields says that in Aristotle's account, a reductive explanation to life is not given [1]. And as we see in the above quote, Aristotle himself believes a living thing (if capable) wants to continue its existence in someway, to partake in the eternal and divine.

How to actualize: being out of equilibrium

Through reading On the Soul, I was struck by how central the question of the soul was for Aristotle as a means to explain what causes life. While science has made many advances on our understanding of how life functions, we still do not fully know how life originated [10], nor do we know about every detail of how biological cells function [7, 8, 11]. Nevertheless, there has still been so much that has been discovered about life's functioning compared to when Aristotle was alive, and I started to wonder whether we could try to reinterpret Aristotle's ideas about the nutritive soul in light of what we now know about living biological cells. There are particularly two areas in science, the study of active matter, and the study of non-equilibrium physics in biology that have contributed a lot to our understanding of how cells function. I'll focus on these two areas and see how the nutritive soul could be reinterpreted in light of what these areas of science tells us about what makes cells come alive. I'll first discuss these two areas which are closely related to each other, and then try to relate them back to the nutritive soul of a cell.

At the microscopic scale, cells are composed of both normal matter, the kind that Aristotle assumed to be devoid of a soul, and also a type of matter called active matter [8]. Of course active matter is also composed of the 'normal matter' of atoms, but active matter can display interesting properties not seen in typical collections of atoms. The most important examples of active matter related to the biology of the cell are molecular motors, which carry in themselves the ability to produce forward or directed movement when supplied with the appropriate chemical energy [8]. If you google 'molecular motors', you will see many images of molecular motors such as kinesin which walks along certain paths in the cell [8]. The cell is full of these molecular motors which are a type of active matter that can originate self-movement [7]. Aristotle attributes self-movement to the appetitive soul [III, 9], which he only thought were seen in animals since he didn't know about cells that can move on their own, and molecular motors are part of the underlying reason for the cell's ability to do so [8].

The study of the role of active matter in biology has shed light on what makes life unique compared to ordinary matter, as well as compared to (present-day) artificial machines. To introduce some technical terminology, life is understood to be driven far from equilibrium, and therefore is considered to be an non-equilibrium system [7]. Active matter are a type of nonequilibrium system [7]. Before explaining what it means to be driven 'far from equilibrium', a system that is at equilibrium basically means there is no change in the macroscopic properties of the system [12]. For example, a cup full of coffee that has cooled down will not suddenly start to warm up again on its own; the coffee cup is at thermal equilibrium with its surroundings. Also, when you pour milk into a cup of coffee, the milk and coffee eventually blend together [12]. This is another type of equilibrium called diffusive equilibrium, where the fluid particles from both the milk and the coffee have blended together and will no longer tend to separate apart into just milk-liquid or coffee-liquid: for example, you would never get the milk just floating to the top of your cup after being well-mixed with the coffee [12].

On the other hand, for a system to be driven far from equilibrium is exactly what it sounds like. Using the previous example of the coffee cup at thermal equilibrium, it would be like the coffee cup all of a sudden starts to warm up again after it had cooled down. The warming up of the coffee obviously doesn't happen on it's own, but the coffee cup does get out of thermal equilibrium when you put the cooled coffee cup into, for example, a microwave oven. Energy from the oven is added to the coffee and eventually warms the coffee up again. Systems that are non-equilibrium typically involve some kind of exchange of energy with its environment [7]. Obviously, living things consume energy from eating food, and they also exchange heat with their environment, making them a system that is far from equilibrium [7].

But being driven far from equilibrium is nothing really unique to life: there are many non-equilibrium phenomena in nature that are not considered to be alive [8]. A system of particles that are driven out of a peaceful equilibrium state by an external source of energy can form an non-equilibrium system in which they self-assemble into a unique pattern that lasts as long as there is an external source that is pumping energy in to the system [8]. Normal machines like a computer, while they are artificially constructed, are also a non-equilibrium system requiring a source of energy [8]. However, what is clear about biological life is that cells and also collections of cells like body tissue are all driven out of equilibrium by thousands these molecular motors that I mentioned about above, each molecular motor being driven out of equilibrium themselves by consuming chemical energy, such as for kinesin, which consuming chemical energy by reacting with adenosine triphosphate (ATP) [7, 8]. Living cells are like a machine made up of machines, to paraphrase Gottfried Wilhelm Leibniz as he put it back in 1704 [8]. Cells are able to hierarchically organize all of these molecular machines in the cell in order to function and maintain itself [4, 8].

Another special property of biological cells is that they are a type of active matter that can self-replicate [11]. While it is not yet known how living organisms manage to integrate other aspects of biological life with the ability to self-replicate [10], there are studies that have looked at how certain non-living chemical species can self-replicate [10]. There are also attempts to make artificial life that can even undergo evolution in the lab [10]. While the possibilities of creating artificial life is open to question, it is clear that life as we know it is still the only example of a kind of active matter that is capable of proliferating through self-replication, what can be called proliferating active matter [11].

Rephrasing Aristotle's Nutritive Soul ...

So I personally think that these are probably the two most relevant new insights that we have gained with regards to the question of what is the cause or source of life, since the time of Aristotle. The underlying physical nature of lifeforms being described as a non-equilibrium system, and the importance of a particular kind of matter that can originate directed movement called active matter. I should mention here that I could totally focused on other topics in cell biology as being the most relevant to our understanding of Aristotle's nutritive soul. So for the purpose of this discussion I'm definitely limiting my scope to what I personally think are the most relevant ideas in cell biology.

So if I were to try my first (insufficient) attempt to rephrase Aristotle's notion of the nutritive soul in a way that would take into account these two aspects, I would think it would be something like, the actualization, as a non-equilibrium system, of a hierarchically organized body of matter that is made of both active and passive matter. The final cause of this actualization is to strive to continue its own existence, which in the end is only physically approximated by self-replication. I think this definition reflects Aristotle's main conception of the nutritive soul, and the only additions that I've made are that I've sort of interpreted 'actualization' as a non-equilibrium phenomena and also added the fact that the body is composed of both passive and active matter. The definition retains the 'non-reductive' character of Aristotle's definition of the nutritive soul since it still states that cells have a final cause to maintain its own existence, something that simple matter, or even a machine on its own may not be said to possess. This property of being directed towards some final end is referred to as being teleological in the philosophy literature [9]. As I mentioned above, there are probably other important aspects in cell biology that I may not have discussed here that I could have included, but I think this definition displays a connection between Aristotle's conception of the nutritive soul and some of our current knowledge about the nature of living cells.

Aristotle's idea of the nutritive soul is very similar in tone to the idea that originates with Kant, which is that the cause of an organisms existence is at least partially internal, and that the organisms components produce and maintain the whole organism [9] This has lead Massio et.al to characterize the self-maintaining aspect of biological life as a system in which 'the activity of the system becomes a necessary (even if, of course, not sufficient) condition for the system itself' [5]. I have not yet studied these ideas in detail yet, but it would be interesting to see how Kant developed his view of biological life, and how Aristotle's understanding of the nutritive soul would be related to it.

I personally think that the very fact that our (created?) physical universe (and its associated laws of physics) allows and maybe generates lifeforms that strive to continue their own existence as something very special and beautiful. We can expect that science will shed more light on the mechanisms that help us understand what gives life or what specific physical conditions are necessary to actualizes the matter of cells. But this all reveals more and more how complex and special the physics and the composition of cells are compared to simple inanimate matter. And it is difficult not to be amazed by Aristotle's clear arguments that inanimate matter would require a very special cause, which he calls the nutritive soul, to be able to come alive.

References

1. Shields, Christopher, "Aristotle’s Psychology", The Stanford Encyclopedia of Philosophy (Winter 2020 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/win2020/entries/aristotle-psychology/>.
2. Ainsworth, Thomas, "Form vs. Matter", The Stanford Encyclopedia of Philosophy (Summer 2020 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/sum2020/entries/form-matter/>.
3. Bechtel, William and Andrew Bollhagen, "Philosophy of Cell Biology", The Stanford Encyclopedia of Philosophy (Winter 2019 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/win2019/entries/cell-biology/>.
4. Fare CM, Villani A, Drake LE, Shorter J. 2021 Higher-order organization of biomolecular condensates. Open Biol. 11: 210137. https://doi.org/10.1098/rsob.210137
5. Matteo Mossio, Cristian Saborido, Alvaro Moreno Bergareche. An Organizational Account of Biological Functions. British Journal for the Philosophy of Science, 2009, 60 (4), pp.813-841. ff10.1093/bjps/axp036ff. ffhalshs-00792047f
6. Alberts, B. (2015). Molecular Biology of the Cell (6th ed.). W.W. Norton & Company. https://doi.org/10.1201/9781315735368
7. Nonequilibrium Physics in Biology
8. Needleman, D., & Dogic, Z. (2017). Active matter at the interface between materials science and cell biology. Nature reviews materials, 2(9), 1-14.
9. WHAT MAKES BIOLOGICAL ORGANISATION TELEOLOGICAL?
10. Adamski, P., Eleveld, M., Sood, A. et al. From self-replication to replicator systems en route to de novo life. Nat Rev Chem 4, 386–403 (2020). https://doi.org/10.1038/s41570-020-0196-x
11. Hallatschek, O., Datta, S.S., Drescher, K. et al. Proliferating active matter. Nat Rev Phys 5, 407–419 (2023). https://doi.org/10.1038/s42254-023-00593-0
12. Schroeder, D. V. (2020). An introduction to thermal physics. Oxford University Press.