The Meaning of "Life"

What conceptual resources are needed for biology? We can gain some insights into this question if we look for an analysis of biology's subject-defining concept, living thing. About 80% of the people with whom I've discussed this believe that living things have a vital essence, in addition to their physical constitution. I'd guess that a lot of you are dualists about living things, too. (Later, when we discuss dualistic theories of thought, you'll see many close analogies between dualism about life and mind. The underlying theme is the way that purpose-directed explanation shapes the debate between reductionists and anti-reductionists. I'll explain that hint near the end of discussion about theories of thought.)

When I suggest that we look for an analysis, I have something specific in mind. An analysis is a definition that provides informative, necessarily true, necessary and sufficient conditions for the application of the term defined. The informativeness requirement rules out circular definitions. The other requirements are sometimes described as "immunity to any possible counterexample." A simple example will illustrate these requirements. Suppose we wanted to define the concept bachelor. We might try this:

The "=def " sign is read "equals by definition" (surprise!), and commits us to maintaining that the following statements must be true:

While the first statement is true, the second is not, as we can show by giving counterexamples to it. Surely some infants are unmarried, but they are not bachelors. Similarly, armadillos are unmarried, but they aren't bachelors either. But Madonna is an unmarried adult human, and she's not a bachelor. So we're missing at least three necessary conditions: being adult and being human. Let's add them:

Now we're committed to the necessary truth of

We can compress these four statements into one:

Suppose we added a requirement:

We'd still have a sufficient condition for bachelorhood, but

is not true. By adding a condition that was not necessary, we'd exclude from the definition a great many things that we ought to include - all the bachelors who were not legal residents of New Jersey.

When I say that they must be true, I mean that not only are there no actual exceptions, but it's not possible that there be any exceptions. Suppose that New Jersey decided to rewrite its tax code to give huge breaks to bachelors, but to impose 95% income taxes on everyone else. Pretty soon, all bachelors would be legal residents of New Jersey and everyone else would move. So it would then be true that

But this would not be a correct analysis of the concept of bachelor, because it would still be possible for someone to be a bachelor and to live somewhere else without losing his bachelorhood. Requiring the necessary truth of the conditions a definition states rules out those odd situations where inessential features of the concept happen to match up with the concept.

Requiring informativeness is important since it's no use to "define a word in terms of itself." If you don't already understand the concept bachelor, it's not going to do you any good to be told that bachelors are characterized by bachelorhood. This kind of blatant circularity is easy to avoid. Sometimes, however, circularity can be buried under many layers of complex definitions and can thereby escape notice until the definition is scrutinized very carefully (if the circle is large, it can be hard to see that it's a circle).

With those preliminaries, we're ready to look for an analysis of "living thing." While the difference between being alive and being dead is a related matter, it's also a different matter, and it's not what we're looking for now.

Two Ancient Definitions of "Living Thing"

Two of the earliest attempts to define "living thing" were made by the ancient Greek philosophers Thales (c600BC) and Aristotle (c340BC). Thales suggested

If Thales is right, then

There appear, however, to be counterexamples to each of these statements. Some living things are (perhaps permanently) paralyzed. This may be too quick, though, since even a paralyzed organism undergoes internal movement. Is internal movement a kind of self-movement? And what exactly does "self" mean here? If a hurricane knocks you down, that's pretty clearly not self-movement. But every movement is related somehow to external forces, since living things are not wholly self-sufficient. Where and how do we draw the line between movement and self-movement? How do we interpret "can" in this context? Are technical assists allowed? - for example, a brain implant that transmits signals to a wheelchair. Suppose that the paralyzed organism while stuck in place could decide to exude a chemical attractant that made neighbors anxious to move it about, but it doesn't know that it has this capacity and never activates it. Is this a kind of self-movement? Perhaps the problem here is not the falsity of the alleged necessary condition, but its lack of informativeness on account of obscurity. And we could imagine a rather simple machine that consisted of a wheeled, motorized cart and an electrical socket sensor: it buzzes about for a while, its battery gets low, it finds a socket, plugs in for a recharge, and is off again. But again, would this be genuine self-movement? Unclarity in the defining concept is a problem again.

Aristotle suggested

But similar problems arise for this definition as well. Does the socket-sucker machine take 'electrical nourishment' from the wall socket? Does it count as a self-nourisher?

Despite their defects, these two ancient definitions share some important features with more modern attempts to analyze living thing. Both definitions say that living things are exactly those things that can perform certain functions. Although these functions are described by words with seemingly clear meanings, their characterizations are unclear, and they in turn require analysis. The characterization of functions often depends on a distinction between the living thing and its environment, which, while commonsensical, stills needs to be drawn precisely. Finally, the sense of "can" in "can perform function F" needs to be nailed down. "Can" means possible, but there are many grades of possibility and we need to discover which of these is appropriate to biology ("biological possibility").

Vitalistic and Mechanistic Intuitions

Historically, there are two divergent kinds of intuitions about the concept of living thing. According to Vitalism, life is essentially mysterious and cannot be created from lifeless matter. The difference between living things and nonliving things (e.g., machines) is a sharp difference in kind, not a merely a difference of degree. So one central vitalistic intuition is that it makes no sense to say that something is more or less a living thing. (Don't confuse this with saying one thing's more alive than another. That usually means the one is younger, more lively and more fun at parties.)

According to Mechanism, however, there's no sharp line between living and nonliving. Rather, living things are just certain complex machines, and the more complex the machine is, the more likely we are to call it a living thing. Since complexity comes in degrees, the difference between living and nonliving would then be a difference in degree. Where the line is drawn is pretty arbitrary, and in the interest of scientific clarity, we should strive to eliminate the concept of living thing in favor of the less arbitrary notions of chemistry and physics.

Naive Vitalism and Naive Mechanism

Two very simple definitions based on these two conflicting sets of intuitions are

Let's consider each in turn.

Naive Vitalism seems to do a good job of reflecting common usage: "He lost his life," "A cat has nine lives," "The corpse was lifeless," "The life has gone out of her." These linguistic data, as well as the writings of some 16th and 17th century scientists (Isaac Newton among them) suggest that vital substance is a sort of fluid with special properties that other fluids could not have (e.g., the power of sensation, the power of self-movement). Two intended consequences of this interpretation are: life could not have evolved from inanimate substances; and no biochemist will be able to create life in the laboratory (unless the supply catalog has a line for vital fluid). Although no one ever refuted this interpretation by producing an example of a living thing bereft of vital fluid, no one ever found any such fluid either, and advances in biochemistry would have been blocked by this interpretation, so it was abandoned.

Naive Mechanism has its problems, too. The definition entails that being a machine is a sufficient for condition for being a living thing. But surely there's some distinction that needs to be drawn between living and nonliving machines, even if it's granted that all living things are machines.

A second consequence of Naive Mechanism is all possible living things are machines. Is this true? Before we can answer, we need to know more about the intended meaning of "machine." Here are two possibilities. One common use of "machine" is to denote a variety of artificial, manufactured, mechanical devices, like sewing machines, bicycles, grandfather clocks and drill presses. Since not all living things are human artifacts, that can't be the intended meaning. A broader notion of machine which seems to have been intended by 15th-18th century advocates of mechanistic intuitions (e.g., Julien de la Mettrie, c1750) is that a machine is whatever can be completely described and explained by the laws of the branch of physics called "mechanics." This is more reasonable than the first meaning, but it seems that no matter how the line is reasonably drawn between mechanical and chemical action, at least some living things must be characterized partly by the latter. So, again, Naive Mechanism fails to give a necessary condition for something's being a living thing.

Perhaps less naive attempts to express vitalistic and mechanistic intuitions will fare better.

Transition to Modern Definitions

It is sometimes suggested that the best way to discover the proper analysis of some important concept is to examine the actual practices of the relevant experts. Since what biologists study are living things, we might say

But this is obviously wrong. Biologists frequently study their checkbooks when trying to balance them, but that doesn't mean checkbooks are living things. The obvious remedy is to tighten the definition by focusing on the things that a biologist studies when she's looking for explanations of biological phenomena. So

This definition is obviously in danger of being uninformative. To remove the danger, we need to know exactly what distinguishes biological from, say chemical or physical explanations. If scientific explanation is ultimately a matter of deploying the relevant laws, then what we need to know is: What is distinctive of the laws of biology, as opposed to the laws of chemistry or of physics? Considering this question will help us to understand modern, more sophisticated descendants of Naive views.

Introducing Organicism and Biological Functionalism

Associated with vitalistic intuitions is the conviction that biology is autonomous: that the laws of biology cannot be explained away as application of the laws of chemistry or physics, because the concepts that are essential to describing and explaining the nature and behavior of living things have no place in chemistry or physics and cannot be given an analysis in terms drawn solely from chemistry or physics. This is just what Organicism, a modern version of Naive Vitalism, holds. (Many would agree that chemical notions can nowadays be analyzed without loss in terms drawn solely from physics, since chemistry is mainly a matter of electron transfer.) Organicists disagree among themselves about which concept is uniquely fundamental to biological explanation. We can focus on one proposal.

Before examining Organicism, let's take a look at its modern, and far more popular, opponent, Biological Functionalism, the modern reworking of mechanistic intuitions. Biological Functionalists agree with Naive Mechanists that the difference between living and nonliving things is a matter of degree, but are far more sophisticated in how they characterize the difference. Roughly, they hold that living things perform functions of far greater complexity and exhibit a higher degree of organization than do nonliving things; and that all such functions can be analyzed in terms drawn solely from chemistry and physics.

Biological Functionalism and Organicism agree that living things have a special kind of organized complexity. While Biological Functionalism claims that this functional complexity can be characterized in terms drawn solely from chemistry and physics (chiefly, the notion of causation), Organicism claims that one must introduce an additional notion of purpose or goal that cannot be so characterized.

Two parallel sets of questions face Biological Functionalism and Organicism. For Biological Functionalism:

For Organicism:

I'll focus first on the second question in each pair and will return to the first questions later.

Two Kinds of Explanation

We have already become acquainted with some purpose-directed explanations in our examinations of UFOlogy, Scientific Creationism and Arguments from Design. Another class of examples can be found in attempts to make sense of people's behavior. You hear that the President is spending lots of money on TV campaign ads, and you are confident that s/he's doing so to get re-elected. To spell things out a bit:

Every one of us runs through such explanations of behavior many times each day, although we're so practiced at doing so that we seldom pay explicit attention to the premises. Like Arguments from Design, these purpose-directed explanations explain through an attribution of purpose. As in Arguments from Design, the conclusions are supported less than conclusively. It's certainly possible for someone to have the specified purpose and the belief about how to achieve it while nevertheless refraining from the described behavior. (This is taken as obvious by all the editorialists who criticize politicians for excessive campaign spending, and who urge them not to act on the implicated beliefs and desires.)

If we wish, we can even bring the second sort of purpose-directed explanation into line with the first sort that we met:

While many living things do not exhibit the same purposiveness as human or divine beings, they do typically have biological survival as their overarching goal, and that goal is key in explaining how their biological structure serves subordinate goals.

It's doubtful that many discoveries about human functional anatomy could have been made without employing some such purpose-directed explanation.

When Harvey discovered the purpose of valves in our blood vessels, he'd have been unable even to guess at the role of those small flaps of tissue in our circulatory system without supposing that they had a subordinate purpose in circulating the blood pumped by the heart. So the utility of purpose-directed explanation in biology seems to be beyond dispute. And biology texts seem to be full of such explanations.

A very different kind of explanation is familiar from even brief forays into the science labs in elementary school. You're in fourth grade and your current science unit is 'the three states of matter' (liquid, solid, gas). Today's experiment is about the transition between liquid and solid states, and you've been given an empty ice cube tray. You fill it with tap water and measure the temperature of the water with your fourth grade thermometer. Then you put the tray in the freezer, wait a while, and repeat your temperature measurement. You find that when the water reaches 0° C (32° F), it's ice. It's a well-known law of fourth grade science that

You can use this general principle, together with your observations of temperature, to account for the water's transition from liquid to solid:

Fourth grade science is, of course, full of falsehoods. But that really doesn't matter very much, because even this 'toy' explanation illustrates the essentials of causal explanation as it appears in more advanced science. We explain by applying a general principle (law of nature) that articulates a physically necessary connection between the occurrence of a cause of a specified type and the type of effect that must follow. Such explanations differ dramatically from purpose-directed explanations since causal explanations make no mention of purposes or goals, and specify an effect that is a physically necessary consequence of the given cause.

Two Analyses of Function

Biological Functionalists don't dispute the utility of "purpose" (or, as they prefer to say, "function") talk, but they are convinced that it's dispensable in favor of better-grounded terms from other branches of science. Since Biological Functionalism maintains that "function" is analyzable causally, they owe us such an analysis. Let's look at two attempts to pay up.

Ernest Nagel once suggested

or, equivalently, p is physically (causally) necessary for O's doing F. For example, this definition entails that "the function of the kidney in humans is the excretion of urine" means: a human can excrete urine only if s/he has a kidney. This definition is, however, open to at least two types of counterexample.

While it's true that my heart has the function of pumping blood around my body, my heart is not necessary for that task. There are a number of artificial devices that will do that, too. So the defintion would say that my heart does not have the function of pumping blood around my body, an incorrect result.

The function of your left ear is to help you to hear, but you could still hear without it, so your left ear is not necessary for your hearing. And in general redundant parts may have functions without being necessary for the functions that they in fact perform.

Our acquaintance from the Arkansas trial, Michael Ruse, proposed instead

While this lacks some of the flaws of Nagel's suggestion, it is also open to counterexample. As Nagel pointed out, it's the function of fur in polar bears to help them keep warm, and if their environment were to heat up drastically, that would still be the function of their fur, but in the changed environment, that function would not contribute to their survival. Contrary to what Ruse suggests, not all biological functions are adaptive. Because functions need not be 'successful', it turns out to be extremely difficult to analyze function purely causally.

Which Functions are Organic?

A remaining question is, Which functions are essential to characterizing each and every living thing? When I've asked people this question, they've relied on their biology texts to give me something like the following list:

F is an organic function if and only if F is either growth, self-reproduction, irritability (ability to respond to stimuli), mobility (self-movement) or metabolism (self-nourishment).

Together with Biological Functionalism, this entails the following six statements:

The last, supposedly sufficient condition could be met by a machine, couldn't it? Would that machine then be a living thing? Until we resolve some of the questions that Thales's and Aristotle's definitions raised, it won't be possible to answer this question.

With respect to the five supposedly necessary conditions for being a living thing: Isn't it possible for a living thing not to perform any one of those functions and still be a living thing? Perhaps it won't live for very long, but won't it still be a living thing for as long as it's around? For example, many organisms are sterile - unable to reproduce - but are still living things.

We might try to get around this problem by modifying Biological Functionalism in this way:

The strategy is to specify some biologically relevant relationship between defective organisms and nondefective, normal organisms by grouping them together in one biological kind. So it is clearly crucial that this revised definition employ just the right notion of kind. The best candidate seems to be "species," a central concept in evolutionary theory. What is the analysis of the concept of species? The concept is defined by the function it performs in evolutionary theory: species is the finest division in the 'organization chart' yielded by evolutionary theory. Which theory is that? Among other things, it is essentially a theory of living things. Buried beneath several layers of definition is the very concept we were trying to analyze. So the modified definition appears to be circular and so uninformative.

Although there are other strategies for dealing with the problem of defective organisms, the difficulties with this strategy should alert us to possibility that specifying normal biological function will be key to the success of Biological Functionalism.

A final remark about Biological Functionalism: if it is function and function alone that is essential to being a living thing, then there's no good reason to think that any particular kind of material stuff (e.g. carbon, oxygen, hydrogen, nitrogen) is required for something's being alive. Any kind of stuff will do, just so long as it's capable of sustaining the organic functions (whatever they turn out to be). Of course, no vital substance is then needed. This raises the possibility of both un-Earthly life-forms and artificial life. UFOlogy gets considerably more complex and subtle if we need to concern ourselves with the former possibility. And if artificial life is possible, can Max Headroom or the Lawnmower Man be far behind?

Additional Sources

2000 David F. Austin

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