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what is life?

by Tam Hunt

What is life? Is life something that, like obscenity, we know 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.


Unfortunately, every definition of life provided thus far runs into serious problems. “Grasping the nature of life is like catching a whirling eddy in a stream: the moment you have it in your hands it disappears and leaves you with the matter but not the form.” 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, including life, are literally impossible. 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.

a history of effort

Numerous modern biologists have attempted to answer the question, What is life? Ironically, the pioneering twentieth-century British biologist J. B. S. Haldane began a short essay titled “What Is Life?” by stating, “I am not going to answer this question.” He recognized the difficulties and stayed away from any definition. There are also three books with the same title that do attempt to answer this eons-old question.

Erwin Schrödinger, a paragon of modern physics and 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), Schrödinger defined life by its ability to create order out of disorder.

This definition is intriguing, but modern knowledge about the self-organizing characteristics of what is normally considered inanimate matter renders it problematic. 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? There isn’t really any clear separation between what is and isn’t negentropic. If life is defined by negentropy, then the whole universe is in some manner negentropic.

Ernst Mayr, an American, was another giant of twentieth-century biology. He taught at Harvard for decades and, after he 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 “a process of living” as opposed to “life”:
  • complexity and organization
  • chemical uniqueness
  • quality
  • uniqueness and variability
  • a genetic program
  • a historical nature
  • natural selection
  • indeterminacy
Unfortunately, Mayr’s system, despite his own cautions, falls into the same trap as that of other biologists. First, his 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 the 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 Dupré 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. I am fine with such an inclusive definition, but most biologists are not. If artificial life is truly to be considered life, then what is the principled distinction between life and nonlife?

Dupré 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, and so on.

This broader problem of attempting to define life becomes even more apparent when we consider the variety of “almost alive” parts of our universe. All of these borderline cases can be described as satisfying the above three-part definition, yet none of them are generally considered by modern biologists to be alive.

almost alive?

Viruses are the most well-known member of this borderline group. They are responsible for the common cold, the flu, and many other diseases. Viruses are very simple creatures that consist of 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 noncomplex 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’s in its dormant state outside of a host cell, to be inert nonliving matter. This is because the virus can’t reproduce itself without invading a host cell. Thus it fails the independent reproduction criterion.

This distinction itself quickly becomes arbitrary, however, when we ponder why the distinction is drawn between a virus outside 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’s 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 a single 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. They 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 they ruin 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. Done at the microbiological level, the act of transferring this information, however, is itself the prion’s reproductive act. Indeed, it is the only reproduction possible for such a simple form, for what else would a prion’s reproduction 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 trying to define what is necessarily in flux. We can solve this problem by suggesting that all things are alive to some degree, where life is simply the flux of increasingly complex forms that include 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. More than two thousand years ago, Aristotle wrote, “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.” Dupré and O’Malley reach the same conclusion in their paper, proposing a continuum approach to life that stresses collaboration.

fine-tuning the continuum 

If you can’t establish where the line of demarcation for life lies, it makes little sense to posit any line at all. With no such line, each particular organism falls on a continuum of having more or less life. All things are organisms in this conception of life. As I’ve written previously, 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 based on 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.”

Only in highly complex collections of matter, such as in biological life (what we generally mean when we talk about life), do we generally 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. 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.’”7 Aristotle, Haldane, Dupré, O’Malley, and Jakosky are not alone among eminent scientists in holding this view. Niels Bohr, the Danish physicist who made seminal contributions to quantum mechanics, stated that the “very definitions of life and mechanics... are ultimately a matter of convenience... The 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 addressed 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.

And here we find 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 that are just a little bit alive. Biology then is 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.

life is... complex

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 forms. Entropy, the second law of thermodynamics, is, in this view, a postulate in the process of being disproven as we realize that life is all-pervasive. 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 an organism dies, its status as a unitary subject disappears even as its constituents may keep on living.)

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 will remain different in practice, having a unifying philosophical framework such as that which I’ve proposed can be helpful in reaching new insights.

About the author:
Tam Hunt is a philosopher, lawyer, and biologist. He lives in Santa Barbara and blogs at Thought, Spirit, Politik at www.tamhunt.blogspot.com.

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