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DOI 10.1007/s11229-011-9873-0
Negotiating boundaries in the definition of life:
Wittgensteinian and Darwinian insights on resolving
conceptual border conflicts
Robert T. Pennock
Received: 24 May 2010 / Accepted: 6 January 2011
© Springer Science+Business Media B.V. 2011
Abstract What is the definition of life? Artificial life environments provide an interesting test case for this classical question. Understanding what such systems can tell
us about biological life requires negotiating the tricky conceptual boundary between
virtual and real life forms. Drawing from Wittgenstein’s analysis of the concept of a
game and a Darwinian insight about classification, I argue that classifying life involves
both causal and pragmatic elements. Rather than searching for a single, sharp definition, these considerations suggest that life is a cluster concept with fuzzy boundaries
and that there are multiple legitimate ways to make the notion precise for different scientific purposes. This pluralist, realist account avoids unnecessary border disputes by
emphasizing how science negotiates such questions in relation to theory and evidence.
I also discuss several objections to this approach, including a “moral hesitation” some
have to allowing broader application of the concept of life to include artificial life.
Keywords Artificial life · Classification · Cluster concept · Darwinian evolution ·
Definition of life · Digital evolution · Pluralism
R. T. Pennock (B)
Lyman Briggs College, Michigan State University,
East Lansing, MI, USA
e-mail: [email protected]
R. T. Pennock
Department of Philosophy, Michigan State University,
East Lansing, MI, USA
R. T. Pennock
Department of Computer Science, Michigan State University,
East Lansing, MI, USA
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1 Prelude: the question of the meaning of “life”
1. Wittgenstein wrote: “Uttering a word is like striking a note on the keyboard of
the imagination” (Wittgenstein 1953, §6). What note is struck when the word “life” is
uttered? Not just one sound, surely. If anything is true of life forms, it is their lush multiplicity. “Life” strikes not just a note, but a full-bodied chord. But what about “artificial
life”? Does this phrase retain the rich sound of the untouched original, or does is strike a
diminished chord? Or would we say that it is only a chord, while life itself produces not
just a chord, but something more like a musical improvisation—theme and variations?
2. In this paper, I will examine the general question of how to define life by
way of the new test case that artificial life (or “ALife”) systems offer. Could an
artificial life system ever qualify as being alive? In discussing whether the term
“life” may properly be applied to ALife systems, I am mostly concerned with
finding a way forward in resolving what seem to be endless debates about the definition
of life. I also hope that this approach will be helpful in answering a controversial question in the field of artificial life research itself, upon which turns the extent to which
ALife systems may serve as scientific models of real biological systems. Finally, a
broader goal is to use this paper as a way to introduce some elements of a theory
of the evolution of terms and concepts in scientific (and other) contexts. My initial
analysis draws inspiration from Wittgenstein’s investigations of language games, but
the eventual resolution comes from a critical insight from Darwin about the nature of
biological classification. Life should be viewed as a fuzzy cluster concept, with multiple legitimate ways of making it precise for specific scientific purposes. The defining
of scientific concepts is a process of negotiation involving both pragmatic scientific
interests and the causal structure of the world.
2 Artificial life
3. Researchers who see their work as falling under the heading of ALife include
biologists, chemists, computer scientists, engineers, philosophers, physicists, and others. Christopher Langton, who coined the term that now embraces the field, explained
artificial life as “The study of man-made systems that exhibit behaviors characteristic
of natural living systems” (1989, p. 1). Some of this work is done in vitro using organic
molecules, some with mechanical systems like robots, some in computers. I will focus
on the last of these, for it is in the in silica research that the concept of life is being
most stretched. As Langton put it elsewhere: “In addition to providing new ways to
study the biological phenomena associated with life here on Earth, life-as-we-know-it,
Artificial Life allows us to extend our studies to the larger domain of ‘bio-logic’ of
possible life, life-as-it-could-be ...” (1992, xviii).
4. It is in these wild places—at the frontiers of possibility—that answers to the question “What is life?” are best tested. We speak today with familiar ease about computer
virtual reality environments, but when do virtual life forms—that at first may only
exhibit the form of living things—cross over to become the real thing? Some ALife
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researchers already attribute life to their digital organisms (Grand 2001). Have they
simply misunderstood the concept? Can the definition of life be stretched this far?
3 Life forms
5. Wittgenstein considered questions about understanding and defining concepts by
examining how to explain the concept of a game. He wrote: “How should we explain
to someone what a game is? I imagine that we should describe games to him, and we
might add: ‘This and similar things are called ‘games”’ (1953, §69).
We could try to explain life in that way, by describing life forms. We could begin
to describe human beings, bison, birds, bees, begonias, bacteria. “These and similar
things are called ‘life”’, we would say.
Conceptual understanding begins by example. Archetypical examples function as
conceptual anchors and become test cases; for any proposed definition to be satisfactory A and B must fall under the definition while Y and Z ought to be ruled out.
Disputes over the definition of life typically revolve around problematic examples that
proposed definitions either bring in or leave out. Are viruses alive? Crystals? Fire?
Thus, one cannot make do with examples alone, if only because it is always reasonable to ask: “Similar? In what respect?”
6. What is the contrast class to “life”? It is of no use to say “The contrast class of
‘life’ is ‘non-life.”’ That trivial logical contrast deals only with the concept of not, and
reveals nothing about the content of the term of interest. Perhaps we mean to contrast
the class of living things to that of the inanimate, or the inorganic. Alternatively, we
might mean to contrast things that live with those that are dead. However, even here
we would need to dig deeper. For instance, are we willing to say of something that “it
is dead” if it never lived? This has been said of the material world, of “lifeless matter.”
However, is that a reasonable use, or do only those who decry the disenchantment of
the world speak in this way?
Whether or not we may reasonably attribute the property of life to ALife will depend
upon the answers to such questions.
7. By what signs do we recognize something as alive? Furthermore, moving to the
higher conceptual level, how do we recognize that a sign is “alive”? Wittgenstein
wrote: “Every sign by itself seems dead. What gives it life?—In use it is alive. Is life
breathed into it there?—Or is the use its life?” (1953, §432).
So, let us first look briefly at how scientists have used the concept of life. Such an
examination will not exhaust this theme of life signs, and we will return to it later, but
we do this as a first approximation.
4 Scientific definitions of life
8. Of the question “What is life?”, Connie Barlow wrote: “The physicist does not
answer the question what an electron really ‘is.’ Likewise, no answer may be expected
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from the biologist to the question of what life may be in its intimate essence” (Barlow
1991, p. 116). But this is too easy. We are not searching for some Platonic essence,
but for something more mundane. Moreover, biologists do regularly propose lists of
defining characteristics of life (though they rarely make clear whether they mean these
to be taken as a set of necessary and sufficient conditions).
Ernst Mayr provides a typical list, citing evolution, self-replication, growth and
differentiation via a genetic program, metabolism, self-regulation, response to stimuli
from the environment, and change in phenotype and genotype as characteristics of
living organisms (1997, p. 22).
Bernhard Rensch had an overlapping, but slightly different set of conditions. He
suggested that life is characterized by reproduction, mutation, variation, constant activity (metabolic functions), comprising certain chemical elements (carbon, phosphorus,
etc.), individualized, self-contained systems (cell membranes necessary), and by purposiveness (1971).
Some focus on a smaller set of conditions. Francis Crick listed replication, mutation, metabolism, and the ability to keep genetic material isolated from its environment
(1981).
9. Others do not presume to offer such a set, but choose to retain an open-ended
notion and focus on what seems to be a key process, usually evolution. Of our question, biologist Richard Dawkins says: “Obviously I do not know but, if I had to bet, I
would put my money on one fundamental principle. This is the law that all life evolves
by the differential survival of replicating entities” (1976, pp. 191–192).
Physicians focus the question differently and consider life in contrast to death, asking what signs distinguish a living body from a dead one. In this context, signs such
as cessation of heartbeat and respiration, which relate to metabolic functioning, have
long been central to the medical definition. More recently the definition has shifted to
cessation of brain function (Beecher 1970), although this is more aimed at identifying
the death of the self for human beings, rather than biological death generally (Pennock
1995).
Physicists also have attempted to answer the question. Edwin Schrödinger famously
proposed that we understand life as a process that decreases entropy (Schrödinger
1944). Even mathematicians have offered some tentative suggestions, such as defining life in terms of Shannon information theory (Chaitin 1979).
It should not be surprising that such definitions differ in part as a function of the
individual research interests of the persons offering them. Indeed, it is part of my thesis
that conceptual variations of this sort are entirely proper.
5 Form of life
10. Wittgenstein noted correctly that there are many different ways that language
is used. “It is easy to imagine a language consisting only of orders and reports in
battle.—Or a language consisting only of questions and expressions for answering
yes and no. And innumerable others.—And to imagine a language means to imagine
a form of life” (1953, §19). Similarly, it is helpful to investigate our general question
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of what life is, and the specific question of whether ALife forms are alive, by thinking
of the forms of life they participate in, especially their uses in scientific contexts. But
before moving to this discussion, we should quickly dismiss an unnecessary worry.
11. As we saw, Wittgenstein explicates his notion by applying it to the concept of a
game. Will we get ourselves into a dangerous circularity by explicating the concept of
life forms by reference to something like forms of life? There is no reason to think we
will. Suffice to say that we are not caught in a trap just because, for instance, we begin
by analyzing the concept “game” and then do so in part by reference to the idea of a
“language-game” as Wittgenstein does—although the same word is used in both, the
concepts, while related, are distinct. When considering language and linguistic signs,
we would be mistaken to look just at their relations to other linguistic objects. We
must also consider language as an activity, and investigate linguistic signs not just in
the abstract but also as they participate in our other activities.
Our interest is not in linguistic usage, but in the scientific articulation of concepts
and terms that relate to hypotheses about features of the world. That is, in analyzing
concepts and constructing definitions here, we are playing a word game, but not just a
word game, for science aims for its terms to refer to empirical reality and not be mere
conventions.
6 Conway’s Game of Life
12. In that spirit, to begin to assess the viability of ALife, let us take John Conway’s
classic Game of Life. Although Conway completed this work before Langton coined
the term ‘ALife’, it is now recognized as a pioneering effort in the field. Artificial life
researchers typically take a “bottom-up” view, holding that the complex, high-level
global patterns of life arise from simple, low-level, local interactions. Conway wanted
to see whether high-level complexities could emerge from a set of simple rules. For
this purpose, he used cellular automata (CA) as a model.
Simple CA are one-dimensional binary strings, but for his set-up Conway used an
unlimited Cartesian two-dimensional grid. A grid cell has a simple binary state and
may be either occupied (on) or empty (off). Cells change all at once from one generation to the next dependent upon the previous status of each cell and its immediate
eight neighbors. Updates follow just two rules:
(i) If a cell is empty in one generation (Gn ), then it remains empty, except if exactly
three of its neighbor cells are occupied: In that case, it will be occupied in the
next generation (Gn+1 ).
(ii) If a cell is occupied in Gn , then it remains occupied whenever exactly two or
three of its neighbor cells are occupied; otherwise it becomes empty in Gn+1 .
One begins with some random or preset initial state of occupied cells, then lets the
cells change generation after generation according to the rules. Over time, the patterns
of the cells change in strikingly complex ways that seem to combine chaos and order
in a sometimes unexpectedly lifelike manner.
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But is the lifelike form of Conway’s Game of Life enough? Suppose someone were
to object: “This is not life; it is not even a game.” Let us take these points in reverse
order.
13. Is Conway’s Game of Life a game? Is this cellular automaton a game? No one
wins or loses; but not all games are competitions of that sort. Some might find that cellular automata leave them cold, but an activity need not be fun to be a game. Conway’s
CA does fall in the center of one common notion of a game—one that Wittgenstein
considered—namely: “A game consists in moving objects about on a surface according
to certain rules…” (1953, §3). Moreover, as Wittgenstein noted of this example, such
board games are just one kind of game. Must a good definition include other kinds,
as well? You could make your definition correct, he said, “by expressly restricting it
to those games” (1953, §3), but it is equally reasonable for certain purposes to use a
different definition or retain a loose notion. So while we might agree that under some
specific definition of ‘game,’ Conway’s is not a game, under other reasonable ones
it surely is. The family resemblance, to use another Wittgensteinian notion, is clear
enough, once it is pointed out.
14. So, we may allow that Conway’s Game of Life is a game, but is it a game of life?
We can approach the latter question in the same way as we approached the former.
Let us first consider a basic point.
What if someone were to say: “Don’t ask me for a definition of ‘life’; I cannot give
you one. But I know it when I see it.” In a famous legal case, Justice Potter Stewart
punted in this way rather than attempting to define pornography. So, is life like pornography—recognizable but “undefinable?” Wittgenstein asked: “What does it mean
to know what a game is? What does it mean, to know it and not be able to say it?”
(1953, §75).
Perhaps it means that the thing is something that we recognize instinctively, so
there is no need for words. This seems implausible for a concept like game, but some
biologists have proposed that life is indeed like this. Neil Campbell, in his classic
biology textbook, introduces the concept in just this kind of way:
Life resists a simple, one-sentence definition because it is associated with numerous emergent properties. Yet almost any child perceives that a dog or a bug or a
tree is alive and a rock is not. We can recognize life without defining it, and we
recognize life by what living things do. (1993, p. 4)
James Lovelock is more explicit: “Our recognition of living things is instant and
automatic.” He goes further, speculating that “we have a rapid, highly efficient life
recognition program in our inherited set of instincts” (Quoted in Barlow 1991, p. 7).
15. Lovelock’s suggestion may seem odd, but it should not be dismissed out of
hand—a reasonable argument can be made for why the ability to recognize living
things would confer a selective advantage and thereby could be an evolved, instinctual
trait. It takes only a little causal knowledge to understand that, in certain situations at
least, organisms that can not distinguish living from dead things will not do as well in
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the games of survival and reproduction as those that can. Thus, an ability to recognize
that distinction in the world and differentially act accordingly would be a target for
natural selection. Of course, one would need to do some empirical work to support this
idea and flesh it out, but it at least has some initial plausibility. In any case, whether
we do have such an instinct, or whether our reactions are only learned, we must still
analyze what other factors may be involved and how these relate to more sophisticated
judgments about the nature of life.
Wittgenstein said something else that is relevant to this idea. He wrote: “Our attitude
to what is alive and to what is dead, is not the same. All our reactions are different”
(1953, §284). Here he was considering the judgments we make about whether a stone
or a corpse could have sensations. That these could be accessible to pain, he judged,
seems as absurd as to think a number could be. But, he continued, “now look at
a wriggling fly and at once these difficulties vanish and pain seems able to get a
foothold here, where before everything was, so to speak, too smooth for it” (1953,
§284).
For our purposes, we may set aside the issue of sensation. (Only living things can
feel pain, but not all living things do.) The first point for us is just that such judgments
become possible only with the recognition of life. The second is that the distinction between living and nonliving or life and death makes a difference to important
behaviors and practices.
16. Wittgenstein continues: “If anyone says: ‘That cannot simply come from the fact
that a living things moves about in such-and-such a way and a dead one not’, then I
want to intimate to him that this is a case of the transition ‘from quantity to quality”’
(1953, §284; emphasis added). Somehow, movement seems relevant to our attributions
of life; movement, at least of a certain kind, is a life sign. Life forms are animated. We
understand the deep etymological roots, by way of the Latin anima and the Sanskrit
aniti, to the archetypical life form—the animal. It seems likely that our most primitive
concept of life is based upon breathing and other distinctive motions. If there is an
evolved instinctual concept of life, it is likely to be based upon recognition of such
patterns.
17. The Game of Life does exhibit a remarkable animation. But are its particular
movements enough like the relevant signs of life? Is the wriggling of the CA sufficient
to make our difficulties vanish? At this point, one can rely only upon a demonstration,
so readers must find a copy of an implementation of the game (freely available online)
to view for themselves.
People disagree in their judgment following this exercise in observation. Some
seem to recognize the inklings of life in these animations. Others judge that these CA
are still too “smooth” for the concept of life to get a footing. I confess that I am not
sure myself what to say, though I have often viewed the playing of the Game of Life.
What does it mean to not even know it?
It may mean that Lovelock is wrong that our recognition is automatic. Or it could
mean that ALife is just new to our planet and the experience of our species and so is
not programmed in our instincts. Or it could mean that the Game of Life is transitional
and is still missing some necessary characteristic of life.
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18. Can we move beyond the automatic-recognition concept of life and try to say
what that missing characteristic is? Perhaps life is not like pornography—unspeakable. We should try to do better. If we consult the definitions offered previously, we
might conclude that a central element that is missing is Darwinian evolution, for that
characteristic is tied to all the proposals outlined above. If this is correct, then we
should say that, for all its tantalizing lifelike patterns, the Game of Life is not a life
form because it lacks particular evolutionary patterns of motion, which many would
say do feel more natural and “organic.”
However, even if we agree with this particular assessment of Conway’s Game
of Life, it does not settle the question for ALife generally, or even for all cellular
automata.
7 Evolutionary computation
19. Although the cellular automata in the Game of Life do not exhibit Darwinian
evolution, it is possible for a cellular automation to do so. This may be done by incorporating a genetic algorithm in the system. Such work with cellular automata has been
done, for example, by Melanie Mitchell and others (Mitchell et al. 1993). But rather
than continuing to use cellular automata as our example, let us consider a different
kind of ALife system that incorporates evolution at a more basic level. Again, there
are many examples that one might pick, but I will take the ALife platform Avida as a
good, representative example.
20. Avida was developed to serve as a platform for controlled experimentation on the
artificial “life forms” that inhabit its environment (Adami 1988). Simply put, in the
virtual environment of computer memory, populations of digital “organisms” compete with each other for space and CPU time, which functions as their “energy.”
The systems are said to have a virtual “chemistry” which is just their particular,
small set of component instructions. Avida has a basic set of about 26 instructions,
which are computationally (Turing) complete. An individual program is simply a
string (of no fixed length) of these instructions. Unlike some ALife systems where
the goal may be to explicitly model, for example, fish or insects, the “organisms” in
Avida are simply the computer programs themselves, and nothing is done to make
them represent particular biological species. However, the instruction set does allow
them to exhibit a key property of biological beings, namely, the ability to self-replicate. In Avida, an original self-replicating “ancestor” program is seeded in the
environment, but thereafter it and its descendents are left to replicate and evolve on
their own.
21. Evolution can happen to the digital organisms because errors are allowed to occur
in the programs. Random mutations (including substitutions, insertions and deletions)
of instructions may occur at any point in the string. If the variants that arise are still
able to self-replicate, those changes will be inherited by their offspring. Natural selection can occur in various ways, such as by having a “reaper” automatically delete
programs that do more poorly than others relative to some measure of fitness, such as
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efficiency of replication. Avida includes a single logical operator—‘nand’ (not-and)—
in its instruction set, and allowing programs to be rewarded that acquire the ability
to perform other, more complex logical operations. Turned loose in their computational environments, over time these digital organisms evolve novel configurations
and behaviors.
8 Negotiating boundaries
22. Wittgenstein wrote: “[H]ow is the concept of a game bounded? What still counts
as a game and what no longer does? Can you give the boundary? No. You can draw
one; for none has so far been drawn” (1953, §68). This last part does not hold true
for the concept of life. As we have seen, there is no lack of “life lines” that have been
drawn. But whose line should we accept?
23. What can we say in general about the drawing of boundaries that might help
us? That boundaries are “socially constructed?” Perhaps in part, but this sort of
analysis only goes so far. That the drawing of boundaries is sometimes done by force?
Certainly (sadly) so, at times. However, is not a diplomatic approach to be preferred?
It is better to rationally negotiate the boundaries of our concepts.
All agree that the exploration of artificial life occurs at the edges of our current
concept of life. How may order be brought to this frontier? In the language-game of
life, are the digital organisms of Avida and other such ALife systems in or out of
bounds?
24. Wittgenstein asked: “[H]ow can I decide what is an essential, and what an inessential, accidental, feature of [some] notation? Is there some reality lying behind the
notation, which shapes its grammar?” (1953, §562). Is it essential in the game of
checkers that players mark the king by putting one piece on top of another? Wittgenstein thought we would agree that this rule is not essential, but offers the suggestion:
“So I am inclined to distinguish between the essential and the inessential in a game
too. The game, one would like to say, has not only rules but also a point” (1953,
§563).
The way we judge a distinction between what is essential and what is inessential will
of course differ when we are considering concepts like games, which are thoroughly
cultural notions, from concepts like life that refer to features of the natural world.
For scientific concepts we do expect there to be a reality that shapes the theoretical
grammar. However, that something is a scientific concept or a natural kind does not
mean that there are no relevant social elements. Wittgenstein is correct that the point,
or purpose, of the concept is relevant.
So, in negotiating the boundaries of life we must consider what purposes the life
sign might serve. Purposes related to biological functions are the center of my interests in this paper, but we should at least acknowledge a couple of other concerns some
people express because of the role that the attribution of life plays in broader social
contexts.
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9 A moral objection?
25. What if one were to object: “These things cannot be life forms, for they are but
automatons, having no value but as entertainment.” We may hear this kind of sentiment expressed by those who dismiss Conway’s Game of Life, Karl Sims’s evolved
virtual creatures, and other forms of ALife as simply pretty pictures. In such objections we find what might seem to be what I call a moral hesitation to broaden the
boundaries of the life. “Life” seems to be regarded as an honorific, and the hesitation
is that to call something a life form would be to recognize in it some intrinsic value
(which might then imply that we could bear responsibilities to it). That would be
as absurd as thinking that the characters in a Disney movie are alive. They are only
animated—animations but without anima.
26. First, as a word of clarification, we must not confuse automatons with cartoons.
In the latter, the movements are at every moment imposed from without, while in
the former they can arise, at least for truly evolving systems like Avida, from within
in the relevant way. I will not repeat here the argument I have made elsewhere, but
the basic point is that evolutionary ALife systems get the causal connections right
(Pennock 2007). They are shaped by random mutation, inheritance and natural selection. In Avida, the organisms contain their own sequences of instructions which control
self-replication. For all the essential Darwinian mechanisms, the causal processes are
hooked up in the right way.
27. As for the moral concern, this deserves more detailed separate treatment. Here
I will just make two brief points. The first is that we should not assume without
argument that all life forms have intrinsic moral value and are deserving of moral
consideration by virtue of being alive alone. For example, we do not in general think
that we have moral responsibilities to individual bacteria, though each is certainly
alive. We thus probably need to consider the question of intrinsic moral value of
life forms on a less general basis, evaluating it in more specific contexts. Given
that, we may make a second point, which is that we might also need to reconsider our initial reaction that it would be absurd to attribute some intrinsic value
to ALife. At present, these automata do not compare even to simple bacteria, but
we would have to reassess our judgment should they evolve to that level of complexity or beyond. Perhaps, at that point, turning them off would indeed kill (sans
scare quotes) them, but that does not mean that doing so would thereby always be
immoral.
10 What about artifice?
28. What if one were to press a different objection: “Automata or not, they are still
only virtual life because they are artificial. That they are the result of artifice disqualifies them from being the real thing.” In digital evolution, at least, this objection
is misplaced, as the specificity of the evolved organism—its particular sequence of
instructions and the functions this produces—is not the result of artifice. The complex
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features of an evolved digital organism are not coded by any programmer, but are
formed by the Darwinian mechanisms themselves.
However, even in cases where artifice is complete or partial, the objection is still
misplaced. We consider ourselves, and the rest of the inhabitants of our biological
world, to be life forms, and would surely not change our assessment should we discover that all life on Earth was originally fashioned from scratch by extra-terrestrial
scientists using genetic engineering. Life that evolves takes on a life of its own.
11 ALife evolves
29. So, is evolution thus a sufficient condition for life, when we turn to biological
considerations? If it is, then ALife forms must be admitted to qualify as alive. As I
have argued elsewhere, ALife systems like Avida do not just simulate evolution; they
are real instances of it (Pennock 2000, 2007).
Although Avida’s digital organisms are simulating the variation of organic organisms, they are not simulating variation—they exhibit real differences. Although they
do not reproduce using the same biochemical processes as organic beings, they do
actually reproduce themselves, and the resulting inheritance of characters from one
generation to the next is no simulation—these are truly passed on. Similarly, although
one might say that the systems are only simulating the struggle for existence of biological organisms, it is nevertheless a real competition and no mere mimic of natural
selection. Thus, in the relevant, Darwinian sense, in studying the descent with modification of artificial life in systems like Avida, scientists are observing evolution itself.
This is what allows them to be used as a true experimental model system to test
evolutionary hypotheses (e.g., Lenski et al. 1993).
30. Such ALife systems that incorporate evolutionary algorithms do not just sound
a single, static chord. They self-replicate. They evolve on their own as organic life
forms do. They exhibit the same sorts of improvisations of themes and variations.
They move in surprisingly familiar and familiarly surprising ways.
31. ALife systems like Avida may even be said to exhibit the purposiveness that some
have listed as an essential characteristic of life. Because their functions truly evolved
by natural selection, they have the same sort of teleological behavior that organic life
has. This and the other emergent features they exhibit deserve separate treatment.
12 Metabolism
32. Of course, not all of the elements that appear on the lists of life signs are realized
in ALife systems. Some are clearly being simulated, and we sometimes note that by
putting terms in scare-quotes. For example, we may speak of digital organisms as
having a “metabolism,” in that they may need to logically process numbers to acquire
additional “energy.” However, these concepts are not instantiated in the model in the
way they are in real life; instead they are represented by means of analogous processes. The “energy” that these digital organisms use is actually central processing
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unit (CPU) cycles, that is, processing time to execute single instructions, which is
distributed by a time-slicing mechanism so that the organisms can operate essentially
in parallel. This is a fair analog, but it is debatable whether these are instances of real
energy uptake by a biological metabolism. All these processes must run in a physical
computer powered by energy, of course, but both conceptual issues and the technical
details of an implementation would have to be examined before one could rigorously
call such a system a realization of a metabolism in the way that they are realizations
of the Darwinian evolutionary processes. I do not have the space to deal with these
interesting complexities in this paper, but it is fair to say that one can imagine how
an artificial system could realize a biological metabolism even if current systems are
judged to fall short.
We may proceed without delving into these issues now, however, to consider a
different possibility: Perhaps having a metabolism, even though it appears on many
lists, is not an essential characteristic of life.
13 Classifying life in a Darwinian mode
33. One might argue: “Evolution is the grand unifying principle of biology, so it
makes sense that it should be regarded as the defining feature of life.” There is much
to be said for this line of argument. If it is use that brings a thing to life, then surely
evolution is the spring of life. It is the Darwinian process, after all, that makes it
sensible to speak of biological functions.
34. But remember that Darwin’s theory was concerned about the origin of species,
not the origin of life. Someone who took the above position that evolution is the
defining characteristic of life might now say, in retrospect, that this was the correct
approach. Evolution would not be relevant to explaining the origin of life except insofar as the two mean the same thing. My own view is that a more general notion of
Darwinian evolution will help explain the origin of life, so I would rather not tie the
two concepts so tightly in this way.
35. Here is one more independent consideration. Even though philosophical arguments and the results in digital evolution that show that ALife systems that incorporate
evolutionary computation are not just simulations, but are instances of evolution, we
still need to reconsider whether evolution is a sufficient condition for life. Binary
strings of a cellular automaton can evolve, so we would indeed have to say that binary
strings of numbers could be alive. Does not that go too far? Pythagoras might disagree,
but most people would say that binary strings of numbers, at least, are too smooth for
that concept to stick.
More could be said here, but I will leave the issue unresolved for the time being
to offer an alternative form of life—a different classification scheme that is equally in
the Darwinian spirit.
36. There is another Darwinian notion that I would argue is more basic and relevant.
It was Darwin’s initial epiphany that opened the door for him to the real possibility
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of descent with modification; namely, that there is no essential difference between
species and varieties. The one concept blurs into the other. Darwin wrote:
I look at the term species as one arbitrarily given, for the sake of convenience, to
a set of individuals closely resembling each other, and that it does not essentially
differ from the term variety, which is given to less distinct and more fluctuating
forms. (Darwin 1964 (1859), p. 46)
This is one of Darwin’s critical insights, as important in its own way, I would argue,
as his discovery of natural selection. It is simultaneously an insight about the nature of
biological entities and also an insight about the nature of classification, which together
turn sharply away from the idea of fixity of biological kinds and show how evolutionary change is possible. However, I do not think that Darwin put this significant point
as well as he might have—terms like species are not entirely “arbitrarily” given, for
such decisions are not a matter of random choice or personal whim. Rather, they are
properly determined, as is always the case in science, by the constraints of empirical
data and scientific theory. Of course, there may be (and indeed are) different, yet still
legitimate scientific species concepts, and different scientists may focus on one rather
than another depending upon their theoretical interests. But this is “arbitrary” only if
one thinks that the contrary means that the decision could not have been otherwise
and that in the end there can be only one correct species theory. A pluralist about species, however, holds that it is perfectly reasonable to have multiple species concepts
depending, pragmatically, upon one’s theoretical interests.
Leaving that caveat aside, Darwin got the basic idea right about the fuzzy distinction between species and varieties, anticipating what Wittgenstein later proposed can
be true of other concepts as well. Wittgenstein wrote: “One might say that the concept
‘game’ is a concept with blurred edges” (1953, §71). So also is the concept life, and
in more than one way. Darwin would certainly agree with Wittgenstein’s recommendation: “Don’t look only for similarities in order to justify a concept, but also for
connexions. The father transmits his name to his son even if the latter is quite unlike
him” (1980, p. 923). This notion of conceptual connection shares much with Darwin’s
evolutionary view, which emphasizes the importance of phylogenetic relationships
over mere morphological similarity for biological classification.
37. Life forms themselves, in Darwin’s sense, we see, are like forms of life in Wittgenstein’s sense. Indeed, Darwin’s tree of life could be taken as an archetypical model
of the notion of family resemblance. I am convinced that Darwin would readily have
accepted the extension of his concept of blurry classification to the question we have
been considering about the definition of life.
14 Classification, pragmatics & negotiation
38. Darwin’s point about species boundaries being “arbitrarily given” may be worded
too strongly, but it retains an important element of truth if one removes the notion of
random personal whim and emphasizes instead the element of judgment by an arbiter.
In this case the arbiter is the scientist who classifies the world with characteristic sci-
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entific ends in mind. The process of conceptual formation and clarification in science
is a kind of arbitration, or negotiation, between the investigator and the world.
Wittgenstein wrote: “Language is an instrument. Its concepts are instruments”
(1953, §569). This pragmatic notion is exactly on target for such conceptual negotiations as we have been considering. The most basic form of life is a negotiation in a
different sense than just the social one that we mentioned before; it is the negotiating
of one’s way around the world.
The world has its own causal structure, and we must negotiate its obstacles. In this
sense, there is “a reality lying behind the notation”; but it is not passive, rather, it helps
shape the language-game. We cannot ignore social factors, but we will fail if we insist
upon negotiating the world on our terms alone. Natural selection will see to that.
We cannot just make of the world what we will, but must meet it half way.
15 The holy grail and the waters of life
39. Mark Bedau has suggested that someone who takes a Wittgensteinian cluster
concept approach is a “skeptic” with regard to the question of whether there is “any
interesting single, all-inclusive account of life” (1996, p. 334). In one sense I accept that
label, though I would put it differently. In arguing for a single, all-inclusive account,
Bedau and others who offer different such definitions are conceptual monists regarding life, whereas I am promoting a form of conceptual pluralism. There are multiple
legitimate ways to define life scientifically depending upon one’s theoretical interests.
More specifically, I am advancing a realist version of pluralism with regard to the
concept of life. Thus, in a different sense I reject the skeptic label, for I hold that life
is a feature of a real, unified world. If life is not a natural kind, then it is hard to know
what would be. But we should not think that natural kinds must have sharp boundaries.
This approach to the definition of life that I have been promoting has the virtue
of explaining why life has vague boundaries and contentious borderline cases. Bedau
and I agree about this. However I reject his main criticism of this sort of approach,
namely, that it makes life seem “arbitrary” or at least “mysterious,” for the same sorts
of reasons that I gave to say why Darwin was wrong to think of species as arbitrarily
given. We can indeed explain why the particular cluster of properties that are associated with life is a fundamental natural phenomenon; this is done in the usual way,
namely, by reference to our best scientific theories, which identify the relevant causal
structures of the world that produce the patterns of interest.
Thus I also disagree with Bedau that this is a “fall back” position. On the contrary,
this view aims for a progressive scientific approach to the question of how to define life
that is responsive to both theory and data, and meshes with how classification works
in other areas in science. It does not presume what is surely an unrealistic requirement
of a sharply-bounded single definition.
40. For similar reasons I differ in part from Cleland and Chyba, who argue that no
definition of life is possible until there is “a general theory of the nature of living things
and their emergence from the physical world” (2002, p. 389). While they are correct
to say that the answer lies in scientific theory, their mistake is in thinking that there
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must be a single all-encompassing explanatory theory of life. That is an unnecessary
search for a holy grail.
On their view, we had to wait until science discovered an adequate chemical theory
before we could define water. But surely that is asking too much of a definition. For
many purposes, even for many scientific purposes, we had a useable concept of water
even before we determined its chemical composition as H2 O. Moreover, were we to
discover at some point that many bodies of water on Earth are actually composed of
both H2 O and XYZ (the substance of Twin Earth fame, which has all the properties
of water but just a different chemical composition) we would not be forced to wait
for a new general chemical theory, but could, for different purposes, make use of both
the old concept as well as some new one without any sacrifice of scientific realism or
rigor. The concept of water is and should be somewhat fluid.
41. The waters of life are of a kind. For many biological questions about life forms,
evolutionary science provides the appropriate explanatory framework, so a definition
of life that focuses on the creative force of evolution is appropriate; thus, Bedau’s
own positive definition of life in terms of evolutionary creativity is useful and proper.
But for other sorts of questions, a researcher may reasonably bracket evolution and
instead focus on proximate causal explanations in terms of metabolic processes. It
may even turn out that there is an ultimate theory of the nature of living things that
goes beyond what we already know about the evolutionary processes that govern their
emergence from the physical world. But we do not need to wait for such a possibility to proceed with the project of definition. And even if such an ultimate theory
were to be found, that would not negate what we have already learned about evolution and the other features of life. A cluster of concepts of life can and will likely
remain.
16 The answer
42. To return to our test case, do the digital organisms of Avida and similar artificial
life systems qualify as life forms, or are they still virtual, having only the outward
appearance of life? The answer, we have seen, depends upon the form of life, in
Wittgenstein’s sense of the term. More precisely, it depends upon scientific theory and
our scientific interests; to judge a possible conceptual answer, we need to know more
precisely what question is being asked. The Darwinian insight about biological classification applies to the definition of life just as it did to the definition of species—in
general, life is too rich to make it reasonable to draw a single sharp boundary. One or
another of the various characteristic features of life that are part of the cluster concept
may be missing in particular cases, and at these fuzzy borders the boundary is defined
by what is being negotiated. Only given some specific question or task does it make
sense to say that something is alive or not, and in those individual cases theory and
data together will provide an answer.
For some such specific purposes, such as investigations of general evolutionary
hypotheses about life, a researcher may find that ALife is the real thing already (e.g.,
we find not a simulation, but a real instance of Darwinian evolution in ALife systems
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like Avida.) But for many other purposes, ALife still has a way to go (e.g., although
a reasonable analog, Avida’s logic-based “metabolism” is not an instance of biological metabolism, and so is not usable as an experimental system for questions about
biochemical life processes.) This approach to defining life explains why there are
contentious border cases and how to go about resolving them. Many unnecessary disputes may be avoided once researchers abandon the misguided search for a single
sharp-edged definition and focus instead on investigating particular variations of the
concept of life in relation to theory and evidence. Recognizing that the borders of life
are fuzzy will also be helpful as one considers any question about how life arises from
non-living things. In the relevant Darwinian sense, life emerges gradually.
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