Humans are blessed with inquisitive minds which attempt to question and logically reason all the phenomena that surround us; from the fall of an apple to the spectacular display of a Mentos in Diet Pepsi. One question, however, has puzzled every mind in history such that with each step we take towards finding an answer, we realize we are a step further from reaching it. That question, crowned “The Ultimate Question”, is

What is Life?

Here, I don’t mean “What is Life?” as in “What is the purpose of my existence?” Rather, I refer to the mysterious phenomenon of nature that created the trees, the bees, the steeds and anemones. At some point in the history of this planet, Life emerged. The general consensus in the scientific community is that the progression from the non-living to the living did not happen overnight, and countless lifetimes’ worth of research have been dedicated to our enlightenment in this process. But how can we study how Life began if we don’t have a rigorous definition of Life in the first place? If the identity of this set of attributes for Life is ambiguous and remains under dispute in the scientific community, how can we sustain the value of our research? This is not to say that there had not been any attempt to solve this puzzle. In fact, great minds in philosophy and sciences have collaborated in this common endeavour. In my brief article, I have gathered some of the best acknowledged theories to date on the definition of Life and exposed the challenges they face in hope to encourage a communal effort to establish a solid answer for this questions that lies at the core of the Life sciences.

First off, to establish a definition in the finicky world of natural sciences, we must clearly specify the intensional and extensional components of the definition. The intensional definition would describe the necessary and sufficient attributes of all members belonging to the defined set. This means that all the attributes in the valid definition are needed to distinguish Life from non-Life, and no additional attributes will be required. To complete the picture, the extensional definition is a set of everything we can consider as Life.

At this point, it’s clear that we cannot gather a set of all members belonging to Life. Instead we must attempt to first establish the intensional definition by stating all of the features of Life until we reach a critical point, the necessary and sufficient attributes, that separate Life from similar things that are not Life.

As mentioned before, the challenge of establishing a definition for Life has not been left unanswered. In fact, the attempt to count all the existing theories for the definition of Life born from all disciplines spanning between theoretical physics, Life sciences, and philosophy, may prove to be just as worthy a challenge on its own. For the sake of discussion and to keep the article’s word count under control, we will need to summarize these into three of the most fundamental theories upon which most of the modern definitions are based: the Cell Theory, the Theory of Negentropy, and the Living Systems Theory.

The Cell Theory is the most popularized theory and is also the first to be taught to budding scientists. The modern Cell Theory founded by Theodore Schwann, Matthias Jakob Schleiden and Rudolf Virchow in 1839, in a nutshell, states that all living things are made of cells, all cells come from pre-existing cells (except for the first cell) and proceeds with what constitutes as a cell. This perspective comes from a structural standpoint where the physical features in common between plants, animals, and microbes act as the differentiator between the living and non-living. Although a very practical tool in the general sense, the limitations to this theory is becoming progressively more evident as more powerful microscopes reveal even smaller things which seem to behave similarly to cells, but are structurally differently.

One of these notorious things is the virus. Like cells, viruses have a bounded environment, have a genetic code, can reproduce genetically similar copies of themselves, and can evolve, among other features. Unlike cells, however, viruses can have RNA instead of DNA as the genetic code, have a protein coat instead of a lipid membrane, and must use a cell’s machinery to reproduce. Whether or not viruses constitute as Life remains a hot topic of discussion, but one thing is certain: according to Cell Theory, viruses are not alive.

At this moment, you may be thinking “But, but, but… Must all Life be composed of cells?” You might even be tempted to quote Shakespeare’s Juliet “What’s in a name? That which we call a rose by any other name would smell as sweet.” Please don’t hesitate, dear reader. All is not lost. It may come as a relief when I tell you that the Cell Theory is not the only theory. This very same type of thinking sparked a cascade of very different theories on the definition of Life.

Erwin Schrodinger’s Theory of Negentropy [1] was published 1944 in a little book titled “What is Life?” Here, he took a very different perspective and emphasized that Life must comply with the laws of physics as it exists in the physical world. The inspiration for this theory sparked from the observation that living organisms possess the ability to remain in a state of order, or low entropy, against the natural tendency for all things to decay into disorder, or high entropy. Schrodinger proposed that living things feed on matter which contain what he calls “negative entropy”, or negentropy, and use them to avoid decay.

However, the phenomenon of resisting decay towards greater entropy can be seen in generally accepted to be non-living materials too. Crystals have the ability to create “order from disorder” and “reproduce” other crystals similar to themselves if a piece of the crystal is placed in a suitable environment. In fact, Schrodinger suggested the existence of a crystal in Life which helps propagate the “genetics” of the Life form. This suggestion is said to have inspired the discovery of DNA later on. The Theory of Negentropy did propose a crucial point that is often overlooked, and that is that however “miraculous” Life may seem, we are still physically composed of nothing more than the same molecules also found in non-living matter. Just as with everything else, these molecules must comply with the laws of the Universe.

In 1978, James Grier Miller published his “Living Systems” theory in a book by the same title. This theory adapts the General Systems Theory, which describes and analyzes a collection of objects that collaborates to accomplish a certain result, into his Living Systems Theory. In Miller’s definition, Life is a concrete (aka: physical) system, which he calls the “Living System”, that exists at seven different levels: Cell, Organ, Organism, Group, Organization, Society, and Supranational System. Within each of these levels of systems, there are nineteen categories of subsystems: Reproducer, Boundary, Ingestor, Distributor, Converter, Producer, Matter-Energy Storage, Extruder, Motor, Supporter, Input Transducer, Internal Transducer, Channel and Net, Decoder, Associator, Memory, Decider, Encoder, and Output Transducer. Living Systems must possess all of these subsystems but more than one subsystem can be manifested into a single physical structure.

The advantage that this theory has over the others is that it emphasizes the principles governing the interactions between components of a system that seem to reoccur at various levels of Living Systems.

Both the Living Systems and Negentropy Theories no longer restrict themselves with the identity of the physical components of living things as in Cell Theory. Rather, they’ve elevated to a more abstract and functional perspective on Life. Abstraction is a powerful tool but can be ambiguous when it comes to distinguishing two very similar things, especially with the extraordinary advancement of technology. Technology is a beautiful and terrible thing. In the recent years, one example of technology has come to dominate our lives, even more so than the government, and has become a leading subject of fascination, fear, and many Hollywood features. I am speaking of course, of the computer. Computer programs are so advanced that we can sometimes confuse the virtual world with reality. Don’t believe me? Just ask any parent of a teenage gamer.

Hypothetically, we can create a computer software that resides in the memory storage of a physical computer system. The software may contain information sufficient to “reproduce” itself and spread to another computer via a network resulting in another computer system behaving in a similar manner. In this hypothetical example, the computer contains all of the components as specified by the Living Systems theory, yet I may speak on behalf of the general populous when I say that this is not an example of a Living system.

Since then, many scientists from across all disciplines have offered their definitions of Life, often taking the form of various combinations of the definitions above. However, it is still difficult to clearly distinguish between crystals, computers, and Life. Here, I offer my opinion on the definition:

Life complies with the disciplines of the General System
but is differentiated by the Natural Reproductive discipline.

Let me explain.

I believe the confusion lies in the fact that these things are made up of essentially the same fundamental units: some nuclei, some electrons and the beautiful molecules they form. So if they’re made of essentially the same things, then the only difference must be in the combinations and interactions between the parts. In other words, system dynamics is the key to why the same fundamental units can create the difference between anything and everything. At this point, it is valid to state that since the fundamental materials composing living and non-living things are the same, a differentiator must be their systematic principles, and Life must be a system.

A system, in essence, is a collection of entities which work as a whole based on a set of principles to achieve a common objective. But not all systems are Living Systems. Now we are in need of another differentiator for living and non-living systems. If we think about it, everything we know of can be thought of as a system. The difference is that they vary greatly in their functions and complexity. The next differentiation we seek, then, is in the function of the system. The General Systems theory upon which the Living Systems theory was based, describes the common elements between all systems: input, output, throughput or process, feedback, control, environment, and goal.

According to Schrodinger’s Theory of Negentropy, the goal of Life is to avoid being at an entropic equilibrium with its environment, at which point the entity is considered “dead”. This suggests that a living entity, as generally accepted, cannot live forever. But, considering that I am alive just as my ancestors once were, Life must have adopted a method of self-replication to allow itself to carry on despite the eventual decay of its physical materials. To distinguish from replication, self-replication is the process by which a system constructs similar copies of itself using its own mechanisms. We should make this distinction because if a potter makes a series of pots based on a template, we can’t say the template performed reproduction. He potter would be the one that performed reproduction and so the function of reproduction can’t be attributed to the pot.

If we look around us, when an animal dies and decomposes, the materials will become part of another living or non-living system. And when that living system dies, its materials will become part of yet another living or non-living system and so forth. What we can deduce is that the materials are relayed between everything, everywhere. What is passed down during self-replication of a living system is not material, but is a set of principles. These principles are the very same ones which governed the “parent” living system. In other words, a living system can be thought of as a set of principles which govern a concrete system, and this set of principles will reproduce itself using the mechanisms of its system.

How, then, can we differentiate artificial computing systems which simulate Life from living systems? The difference here is that artificial systems follow artificial principles whereas living systems follow natural principles. The philosopher Karl Popper noted that artificial principles are subject to what people think while natural principles are independent of what we think. The principles that tell a software how to “behave”, “reproduce”, and “grow” are completely dictated by people while all principles governing living systems, including their reproductive principles, are governed by nature and cannot be altered by human involvement. This is not to say that humans cannot interfere with living systems and alter their normal development. It is possible for humans to create offspring in vitro, or create genetically modified animals, but in order for these particular living systems to live, they must still comply with a set of fundamental rules set by nature.

As I conclude this article, I’m beginning to realize that this fundamental principle governing Life that I spoke of may never be discovered. But just like opening a Kinder Surprise, it’s the journey through the milky chocolate layer and the struggle with the resistant plastic casing that brings the sweetest excitement. Some people say that “Life is whatever you want it to be.” In a way, it’s painfully true. The universe, in all its glory, will always be an unsolved mystery. However we try to put it into words it will always be just that, our words, which can be written and erased and re-written. But a good attempt is always valuable and each step we take towards learning the meaning of Life is a step forward in our exploration through the mysteries of the universe.


1. The original term used by Schrodinger was “negative entropy”. The term was later shortened into “Negentropy” by Leon Brillouin.