Today's guest post on defining life, nicely follows Fernando Baquero's "Organs Before Organisms" post. Today's guest authors are Chelsi Cassilly, planetary protection engineer at NASA, and Eric Bodlak, aerospace-engineer-turned-philosopher. I thoroughly enjoyed editing these two start-of-the-year posts; I hope you will enjoy them as well. Both pieces beautifully exemplify what I'm hoping to provide more of in STC's future, as I mentioned in my post of Monday, January 20. – Roberto
by Chelsi Cassilly and Eric Bodlak
What is life? Initially, it seems like a simple question. The immediate response is often to compare and contrast the many things we encounter daily that are animate and inanimate. Yet this question becomes harder the longer you sit with it. Because life on this planet – that is, life as we know it – includes microbial life, which can strip the complexity of multicellularity away, leaving us with a very broad, objective concept of life, increasingly complicating the task of defining what "it" is that all lifeforms share.
In some countries, the acronym MRS GREN is still used to determine if something is living. It stands for: movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition. However, in the scientific community, there have been debates on this topic leading to papers describing plausible definitions of life, none of which are universally accepted. These studies point out the key features present in living organisms, which consist of: ordered structure, reproduction, growth and development, energy utilization, response to the environment, homeostasis, and evolutionary adaptation (Campbell and Reece, 2002). Perhaps an addition to this list is the ability to continually resist entropy. The Astrobiology Program at NASA still uses the definition of life as put forth by Joyce et al. in the foreword of the 1994 book Origins of Life: "a self-sustaining chemical system capable of Darwinian evolution."
However, the criteria contained in these definitions and characteristics are not without some exceptions. The classic example of an outlier is a virus. Viruses are obviously chemical in nature, typically composed of proteins and nucleic acids. They are also capable of evolution; however, they are not self-sustaining, as they require a host cell for reproduction. Some in the scientific community have argued that viruses are alive, but this is far from universally accepted. (Barring the addition to the tree of life, this raises the obvious question: then what are viruses?)
Furthermore, there are instances of bacteria (e.g. Chlamydia trachomatis) and parasites (e.g. Leishmania) as obligate intracellular pathogens, meaning that they, too, are not exactly self-sustaining and require at least part of their life cycle to occur within a host cell. And certainly, as time goes on, we are appreciating more and more that even higher-order organisms would not survive without commensal microbial communities, bringing to question what it truly means to be "self" sustaining.
Finally, many organisms go dormant, such as endospores or tardigrades, which may exhibit no signs of life during these periods or conditions other than being a chemical system.
In addition, other cases exist where clearly non-living materials exhibit some of the key features defined above. Such considerations would include crystals which defy entropy in their natural order, and micelles, which naturally form given their environmental conditions. Additionally, prions are misfolded proteins that infect mammals and, in a sense, "reproduce" themselves by inducing the production of more prions.
Robotic systems are also an example of an ordered system that utilizes energy and responds to its environment. More recently, and to a striking degree, artificial intelligence has demonstrated the added ability to learn and adapt behaviors, which might be seen as a form of evolution.
For these reasons, it is clear that defining life is anything but simple. Likely, this is a question that science cannot answer, at least not alone.
In light of the predictive successes and technological outworkings of modern science, it is tempting to believe that science alone can provide objective knowledge and that any questions outside its purview are meaningless or unanswerable. However, this seemingly straightforward view is false. For one thing, the idea that science is the only path to knowledge cannot be demonstrated using scientific methods and thus lies outside science's scope. It is, in fact, a philosophical judgment that must be defended using the tools of philosophy, not science.
Further, when scientists interpret empirical data, they do so within an implicit metaphysics or philosophy of nature, which answers questions about the uniformity, stability, and intelligibility of the natural world. It is often supposed that the higher sciences are reducible to the truths of mathematical physics, which is the gold standard for scientific precision. Whether this is true or not, it is again a philosophical (rather than scientific) conclusion, one of which contemporary philosophers of chemistry and biology have yet to settle decisively.
Perhaps surprisingly, the resulting trends towards concepts like emergence and downward causation, as well as a renewed debate over teleology, point toward an Aristotelian philosophy of nature, which conceives of living things as goal-directed wholes, not mere collections of parts. Aristotle understood life as the capacity for "self-motion,"* by which living things act on themselves from within to bring about the effects characteristic of their natures. (Examples include nutrition, sensation, and thought.)
This deceptively simple framework has the potential to anchor wide-ranging judgments about what constitutes life or not. For instance, although biologists are not concerned with metaphysical speculation about beings like God or angels, Aristotle's definition can make sense of the idea that if such beings exist, they are alive, albeit in a non-physical way. More practically, insofar as the activities of robots or artificial intelligence can be reduced to the collective actions of their parts and programming, they possess no intrinsic unity within which to act as a whole. They cannot "move" or actualize themselves as life does, even if they simulate various aspects of life. Viruses, prions, crystals, fire, etc., may adapt evolutionarily, reproduce, grow, or metabolize, but are, nevertheless, not self-movers, instead relying solely on external causes for these lifelike activities. This indicates that many of the expected markers of life are not definitive, at least when broadly construed. On the other hand, Aristotle's approach is open wide to the possibility of lifeforms that are very different from life on Earth and yet living even so.
Ultimately, whether or not Aristotle's conception of life is the right one, it can be used to illustrate the necessary partnership between science and philosophy. Here, the role of the philosopher is to define life (in dialogue with the scientist), and the role of the scientist is to investigate, in turn, the fit of observable entities with the philosopher's definition.
*This term is a summary of Aristotle's views used by later commentators, not his own explicit formulation.
Dr. Chelsi Cassilly attended Lipscomb University in Nashville, Tennessee where she obtained her bachelor’s degree. She earned her doctorate at the University of Tennessee, Knoxville in the lab of Professor Todd Reynolds. She conducted postdoctoral work at Harvard Medical School with Professor Jon Clardy. Currently, she is a planetary protection and materials engineer at NASA Marshall Space Flight Center in Huntsville, Alabama.
Eric Bodlak received his bachelor's degree from Iowa State University in Ames, Iowa. He then completed a masters degree in aerospace engineering from the University of Kansas. He is currently a research engineer at the University of Alabama in Huntsville and working toward a Ph.D. in philosophy. He is originally from Nebraska, but has found many compelling reasons (one being coauthor) to stay in the south.
Do you want to comment on this post? We would be delighted to hear from you! Please send us an email, or comment on Mastodon or Bluesky. Feel free to subscribe if you'd like to receive regular email notifications about new blog posts.
Comments Arriving
From Fernando Baquero (who authored our prior Monday post): "On the Plurality of Life: Small and Big Things Considered" I fear that the original sin of our thoughts about the origin of life is to consider that what we consider life is the only possible life. The question, paraphrasing Schrödinger, is not "What is life?" but "What are lives?" My feeling is that the "what is life" question weakens the general notion of life, which, therefore, cannot rightly be defined out of the limits of the type of life that we recognize as life. Possibly, lives are different forms of existence that can be imagined as resulting from existential forces, providing the conditions to exist, that is, to persist during a quantum of time, considered as the minimal unit of time needed to exist. In this view, provided such minimal ontological requirement, everything that exists is alive. The Heideggerian notion of Dasein (being there) is not only applicable to human beings but to anything that exists. In short, Life is Being. Not very far from the Aristotelian view.
Comments