What Is Life?

Science … is becoming the study of organisms. Biology is the study of the larger organisms; whereas physics is the study of the smaller organisms.~Alfred North Whitehead, Science and the Modern World (1925)

What is life? Do we, as with art and obscenity, “know it when we see it?” This intuitive approach may be good enough for many people, but science seeks definitions in order to get a better handle on the phenomena being studied.

The last couple of essays in this series have discussed theories of evolution without stopping to establish what the heck we are talking about in discussing “life.”

Unfortunately, every definition of life provided thus far runs into serious problems. Aristotle perhaps said it best: “Nothing is true of that which is changing.” In other words, if all is in flux – as all things are – then static definitions of physical phenomena are literally impossible, including life. This is a fundamental limitation that is too rarely acknowledged in modern science and philosophy. We may carve out generally workable definitions, as rules of thumb (heuristics) for deeper study, but we must always acknowledge that any definition regarding physical phenomena that ignores the truth of flux fails from the outset.

Numerous modern biologists have attempted to answer the question: What is life? J.B.S. Haldane, the 20th Century British biologist, a giant in his field, began a short essay – “What is Life”? – by stating, however: “I am not going to answer this question.” He recognized the difficulties and stayed away from any definition. There are also three books from the 20th Century alone, with the same title, which do attempt to answer this eons-old question.

Erwin Schrödinger, a paragon of modern physics well-known for his role in shaping quantum theory, described in his little 1935 masterpiece What is Life? the concept of negative entropy, or negentropy, as the defining characteristic of life. Contrary to the Second Law of Thermodynamics, which asserts that the general tendency in our universe is for order to decay into disorder – entropy – the tendency of life, indeed the very defining characteristic of life, is the opposite. Schrödinger defines life by its ability to create order out of disorder, to defy the trends that inanimate matter must otherwise inexorably follow.

This definition is intriguing, but modern knowledge about the self-organizing characteristics of what is normally considered inanimate matter renders it problematic as a definition of life as something distinct and qualitatively different from non-living matter. When water freezes it transitions from a less ordered state to a more ordered state. This is negentropy. But is water alive? What about crystals more generally, whether of water, silicon or metal? We shall see below that there isn’t really any clear separation from what is negentropic and what is not. If life is defined as what is negentropic, then the whole universe is in some manner negentropic because key parts of the universe are negentropic; and perhaps, over time, the whole universe will become negentropic. Hold that thought.

Ernst Mayr, an American, was another giant of 20th Century biology. He taught at Harvard for decades and, after he had retired, wrote his encyclopedic overview of biology, The Growth of Biological Thought, and many other books. He acknowledged the difficulty in defining life: “Attempts have been made again and again to define ‘life.’ These endeavors are rather futile since it is now clear that there is no special substance, object, or force that can be identified with life.”

Mayr couldn’t resist however, proposing his own list of criteria to describe the “process of living,” as opposed to “life.” Mayr’s criteria for living processes were:

~complexity and organization
~chemical uniqueness
~quality
~uniqueness and variability
~a genetic program
~a historical nature
~natural selection
~indeterminacy

I won’t go into details regarding Mayr’s system except to say that Mayr, despite his own cautions, falls right into the same trap as other biologists, with his criteria for living processes, that he warned about in refusing to define “life.” First, Mayr’s criteria are, collectively, a definition of life – which he said he wasn’t going to provide.

Second, all of Mayr’s criteria either fall on a continuum or are arbitrary distinctions proposed intuitively and without a deeper foundational principle. Why must life have a genetic program, and what does this even mean? Does the genetic program have to be DNA? Can it be bits of code in a computer? Mayr’s writings on these questions reveal his own lack of resolve on this topic. He suggests that computers and software may contain instructions akin to DNA, but then fails to explain why software “DNA” is qualitatively different than non-software DNA. The same can be said with respect to all of his criteria.

A simpler definition of life is offered by British biologists John Dupre and Maureen A. O’Malley. They discuss the three criteria for life that most modern approaches to defining or characterizing life include:

~reproduction
~metabolism
~spatial boundedness

This definition of life gives rise to the possibility that mechanical or electronic creatures may be considered alive, assuming such creatures will eventually be able to reproduce themselves, as they surely will be able to do in coming years. Personally, I am fine with such an inclusive definition, but most biologists it seems are not. If artificial life is truly to be considered life, then what is the principled distinction between life and non-life?

Dupre and O’Malley raise additional problems with these criteria, including the key fact that almost all organisms rely on other organisms for metabolism and reproduction, challenging the notion that we can point to a particular organism and call it “alive” and pretend that it is entirely distinct from its network of symbionts, parasites, etc.

This broader problem with all attempts to answer “what is life?” becomes even more apparent when we consider the variety of “almost alive” parts of our universe. All of these border-line cases, described below, can be described as satisfying the above three-part definition. Yet none of these borderline cases is generally considered, by modern biologists, to be alive, revealing the problematic approach to “life” that is implicit in today’s biology and philosophy of biology.

Viruses are the most well-known member of this group of borderline cases. Viruses are responsible for the common cold and for the flu, as well as many other damaging diseases. Viruses are very simple creatures that consist of merely a protein shell and a dab of RNA, which is a precursor to DNA. Viruses can’t reproduce without invading host cells and co-opting their reproductive machinery. A virus will attach itself to a cell wall, penetrate the wall and transfer its RNA into the cell. The RNA melds itself with the cell’s DNA, forcing the cell to create more viruses. It’s incredibly ingenious when we look at it with fresh eyes. How on Earth did such complex processes evolve in such tiny and apparently non-complex creatures? It’s one of many marvels of life as we know it.

Yet many biologists consider viruses not to be alive. Or, to be more accurate, they consider a virus when it is in its dormant state outside of a host cell to be inert non-living matter. This is the case because the virus can’t reproduce itself without invading a host cell. Thus it fails the independent reproduction criterion.

This distinction itself quickly becomes manifestly arbitrary, however, when we ponder why the distinction is drawn between a virus outside of a cell and a virus inside a cell. Once the virus is inside the cell, it loses any independent existence because its RNA melds with the DNA of the host cell. If the virus outside of the cell, with its little protein shell and RNA, is not alive, what suddenly becomes alive when it merges with the host cell? Is it now a virus/host combination entity that is alive? Or is the virus to be considered conceptually distinct even when it is attached to a host cell and its RNA injected into the host cell? If so, why? And at what exact point does the virus suddenly become “alive” as it attaches to a cell and injects its RNA?

Self-replicating RNA is a second type of borderline biological agent. Self-replicating RNA consists of only a strand of RNA. As the name suggests, it’s different than normal RNA, which occurs inside cells, in that it can reproduce itself without a cell’s help. Self-replicating RNA creates whole new strands of RNA as a free-floating agent outside of a cell. Is this life? Why not?

What about prions? Prions are self-replicating molecules responsible for various diseases such as “mad cow disease.” Prions are even simpler than viruses and self-replicating RNA. Prions consist of nothing more than a very simple protein enfolded in a certain way. In fact, some definitions of “prion” refer only to the information about enfolding the protein, rather than the actual protein. Prions – a contraction of “protein infection” – infect normal proteins and cause them to fold in a way that is always lethal. In cows, the prion infects the brain and causes normal proteins to fold in such a way that it ruins the normal functioning of infected cells. Prions are like viruses in that they don’t seem to have built-in reproductive machinery (and if the prion is simply information that directs the enfolding process it doesn’t, by definition, have any “machinery” at all).

Prion reproduction is a simple transfer of information, consisting of the way the infected protein folds, from a prion to a normal protein. The act of transferring this information, however this is done at the microbiological level, is itself the prion’s reproductive act. Indeed, it is the only reproduction possible for such a simple form, for what else would reproduction of a prion, a mere way of protein enfolding, consist of? We see, then, that the prion does in fact have its own reproductive machinery built into its very simple structure. Recent research has also found that prions evolve just like DNA-based life. So is a prion alive? If not, why not? It seems to meet the three-part test.

This is the kind of difficulty that arises from any proposed definition of what is necessarily in flux: life or even the “process of life” that Mayr tried to characterize with his criteria. We can solve this problem by suggesting that all things are alive to some degree. Life is simply the flux of increasingly complex forms, which includes all matter in the universe. As matter becomes more complex, it becomes “more alive.” An electron is alive, but just a tiny bit. A molecule of oxygen is alive, but just a little bit. A virus outside a cell is alive, but just a tiny bit, and a prion, and so on. Aristotle wrote, two and a half thousand years ago: “Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation, nor on which side thereof an intermediate form should lie.” Dupre and O’Malley reach the same conclusion in their paper, proposing a continuum approach to life that stresses collaboration.

If you can’t establish where the line of demarcation lies it makes little sense to posit any line at all. With no line, life becomes a continuum of more or less life in each particular organism. And all things are “organisms” in this conception of life. As we’ve seen in previous essays, Whitehead conceived of all matter as “drops of experience.” A key feature (perhaps the feature) of this rudimentary experience is will, which includes at its most fundamental level the ability to make choices about how to move and how to manifest in each moment, given the tumult of available information from the surrounding universe. Whitehead, Schopenhauer, David Bohm, Freeman Dyson, David Ray Griffin, and others have suggested that all matter, even subatomic particles, has some freedom of choice over how to move and manifest in each moment. Dyson writes that “the processes of human consciousness differ only in degree but not in kind from the processes of choice between quantum states which we call ‘chance’ when made by electrons.”

It is generally only in highly complex collections of matter, such as in forms that we consider alive from an intuitive point of view, that we see the obvious manifestations of this ability to make choices. But the choices are also manifest, as Dyson writes, in forms that we would not traditionally consider alive, such as atoms and subatomic particles.

J.B.S. Haldane, who puckishly refused to answer the question of what life is in his 1947 essay, supported the view that there is no clear demarcation line between what is alive and what is not: “We do not find obvious evidence of life or mind in so-called inert matter…; but if the scientific point of view is correct, we shall ultimately find them, at least in rudimentary form, all through the universe.”

More recently, University of Colorado astrobiologist Bruce Jakosky, who has worked with NASA in the search for extraterrestrial life, asked rhetorically: “Was there a distinct moment when Earth went from having no life to having life, as if a switch were flipped? The answer is ‘probably not.’” Aristotle, Haldane, Dupre, O’Malley and Jakosky are not alone, however, among eminent scientists in holding this view. Bohr, the Danish physicist who made seminal contributions to quantum mechanics, agreed, stating that the “very definitions of life and mechanics … are ultimately a matter of convenience…. [T]he question of a limitation of physics in biology would lose any meaning if, instead of distinguishing between living organisms and inanimate bodies, we extended the idea of life to all natural phenomena.”

This argument shares many obvious similarities with the argument for panpsychism in earlier essays in this series – the idea that all things have some type of experience that becomes more complex as the organization of matter becomes more complex. We see now that “life” and “consciousness” may be viewed as different terms for the same phenomenon. As matter becomes more complex, it becomes more alive and more conscious. These are simply two ways of saying the same thing.

We reach, with this analysis, a smooth synthesis of physics and biology – as Whitehead suggests in the quote at the beginning of this essay. Physics is the science of fundamental physical forms, organisms, which are just a little bit alive. Biology is, then, simply the science of more complex organisms. The practical dividing line between these two fields becomes arbitrary and a matter of convenience. There is no real dividing line at all.

So what is life? We are led in the final analysis to realize that Schrödinger was right in his assertion that the defining characteristic of life is negentropy – a tendency toward order, toward form. Life is the universal process of creating and maintaining new forms, instead of the opposite tendency to destroy form. Entropy, the second law of thermodynamics is, in this view, a postulate that is in the process of being disproven as we realize that life is all-pervasive. This is another way of stating Schrödinger’s insights. This view of life as universal is known as hylozoism or panzoism.

Life is simply shorthand for the complexity of matter and mind – which are two aspects of the same thing. That is, each real thing is both matter and mind. We apply the label of “life” as a matter of convenience to more complex forms of matter and mind. But there is no point at which a particular collection of matter suddenly becomes alive. Life does not “emerge.” (Life does, however, disappear rather suddenly, from particular organisms, as we are reminded all too often. Death becomes, in this view of life, a matter of different levels of organization: when a given organism dies, its status as a unitary subject, a unitary organism, disappears even as its constituents may keep on living. I will be fleshing out my thoughts on death in later essays.)

This view of life and consciousness as two terms for the same phenomenon provides a unifying framework for physics, biology and the study of consciousness. While the particular tools for studying phenomena within each field as we know it today will remain different in practice, having a unifying philosophical framework can be helpful in reaching new insights.