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Can we define a set of necessary conditions for something's being alive?

My stipulation of a concrete material realisation as a necessary condition for being alive harks back to Aristotle, who, it will be recalled, refers (De Anima 2.1, 412a20) to the soul as "the form of a natural body" (1986, p. 157, italics mine). As he puts it (De Anima 2.1, 412a19):

For the body, far from being one of the things said of a subject, stands rather itself as subject and is matter (1986, p. 157).

The importance of a concrete material realisation is that it individuates an entity, making it a single subject. Without a material realisation, the entity, having no spatio-temporal location, could not properly be described as a single individual. An entity with purely formal properties would lack a specific "thisness" about it. Lacking individuality, it could not meaningfully be described as benefiting or as being harmed, as no specific (spatio-temporal) events could be said to affect it. Without the possibility of ascribing benefit or harm to such an entity, there is no way in which it could be said to possess a telos of its own. Such an entity would therefore lack intrinsic finality.

The implication of this is that a piece of computer software, even if it realised the properties of nested organisation and a master program, could not be said to be alive, as its realisation is purely formal and not material. Hardware is an essential part of what it is to be alive. Living things, after all, are made of flesh and blood, not memes (pace Richard Dawkins).

(Some philosophers would argue that restricting the attribution of "life" to material entities is unduly narrow, as it would rule out the possibility of anything transcendent being alive - God, for instance. I do not propose to discuss this idea within my thesis, as the term "alive", when predicated of anything which is not corporeal, is obviously being employed analogically, not literally. My concern is with the biological property of being alive, which, I have argued, is tied to the Aristotelian property of intrinsic finality.)

What sort of concrete material realisation does a body need in order to qualify as being alive? Humphrey (1993, p. 194) ties the possibility of consciousness to the possession of an intrinsic boundary by an individual. Without such a boundary, argues Humphrey, it could not feel anything happening to it and hence would have nothing to be conscious of. I would go further and suggest that nothing can be said to be alive without an intrinsic boundary. There are two reasons why having a boundary is important. First, an organism that lacked a boundary could not distinguish itself from other individuals, and would thereby be rendered incapable of advancing or defending its own intrinsic ends. For instance, it could not "defend itself against injury" (Koshland, 2002, p. 2215). Second, an organism needs a boundary to contain whatever parts and systems it needs to maintain itself. Koshland spells out the rationale in detail:

All the organisms that we consider living are confined to a limited volume, surrounded by a surface that we call a membrane or skin that keeps the ingredients in a defined volume and keeps deleterious chemicals - toxic or diluting - on the outside. Moreover, as organisms become large, they are divided into smaller compartments, which we call cells (or organs, that is, groups of cells), in order to centralize and specialize certain functions within the larger organism. The reason for compartmentalization is that life depends on the reaction kinetics of its ingredients, the substrates and catalysts (enzymes) of the living system. Those kinetics depend on the concentrations of the ingredients. Simple dilution of the contents of a cell kills it because of the decrease in concentration of the contents, even though all the chemicals remain as active as before dilution. So a container is essential to maintain the concentrations and arrangement of the interior of the living organism and to provide protection from the outside (2002, pp. 2215-2216).

While endorsing the thrust of Koshland's arguments, I should point out that he is assuming that living things are held together by chemical reactions. I would urge caution here: we cannot exclude the possibility that there may be an organism whose internal organisation is maintained by the transmission of, say, electromagnetic signals between its components. Such an organism could survive as long as its parts were suitably configured for sending and receiving signals. Indeed, in different environments, the organism might change its underlying hardware completely, taking in all kinds of "nutrients" whose sole common property was that they were suitable for being configured into the electrical circuitry required to maintain the organism. A von Neumann probe, described in a thought experiment by the physicist Frank Tipler, could operate in such a fashion. As Tipler conceived it, such a probe could travel around the Galaxy, guided by an in-built computer "capable of self-replication and capable ... of constructing anything for which it has plans, using the raw materials available in the solar system it is aimed at" (1982, p. 34). If such a machine could be constructed in such a way that its parts had internal relations, dedicated functionality and a nested hierarchy of organisation (required for intrinsic finality) then it could have multiple chemical "realisations", but would still qualify as being alive. However, even an organism constructed in this way would still need an internal boundary of some sort, to protect itself from the elements and to contain its vital circuitry.

At this stage it might be asked whether an organism needs to be spatially contiguous. Could it have parts that were physically separated, but which were able to communicate with each other by transmitting chemical or electromagnetic signals? The idea is not a new one, as Holldobler and Wilson write:

The idea - the dream - of the superorganism was extremely popular in the early part of this [20th] century. William Morton Smith, like many of his contemporaries, returned to it repeatedly in his writings. In his celebrated essay, 'The Ant Colony as an Organism', he stated that the animal colony is really an organism and not merely an analog of one. It behaves, he said, as a unit. It possesses distinctive properties of size, behavior, and oragnization that are transmitted from one generation to the next. The queen is the reproductive organ, the workers the supporting brain, heart, gut, and other tissues. The exchange of liquid food among the colony members is the equivalent of the circulation of blood and lymph (1994, p. 110, italics mine).

Ants communicate with each other chemically, via pheromones, so the unity of the colony is maintained. The question of whether an ant colony is really one organism will be discussed in a later chapter, but the illustration has a point. There seems to be nothing inherently impossible in the idea of a spatially discontiguous organism. The conceivability of such an organism, however, in no way guarantees its real possibility. Moreover, there are two problems with the idea that need to be addressed: first, in what sense could a disconnected organism be described as one (the "unity" problem), and second, how could two such organisms be distinguished from one another (the "individuation" problem)?

I would suggest that the solution to both problems lies in Aristotle's notion of intrinsic finality and formal causality, recast as a nested hierarchy of dedicated functionality and a master program. I would propose that if:

(i) the parts of a superorganism, although physically separated, are controlled by the same master program, so that they are guaranteed to work together for the good of the whole;

(ii) there is a nested hierarchy of functionality;

(iii) the repertoire of the parts' functionality is dedicated to supporting the functionality of the whole to which they belong; and

(iv) none of these parts is capable of surviving on its own,

then there is no real reason to ascribe the parts a separate telos of their own, as their ends are completely subsumed within that of the extended organism to which they belong. The above conditions are, I believe, sufficient for the existence of a superorganism, and the first three conditions are certainly necessary.

It needs to be stressed that the parts of a superorganism would need to be directed by a master program. Mere co-operation between distinct organisms, as is seen for example in symbiosis, does not suffice to make a superorganism. For example, some insects that feed on plants give sugary secretions to ants for food, and in return they are protected from enemies and sometimes even accepted as virtual members of the colony - a variety of symbiosis known as trophobiosis (Holldobler and Wilson, 1994, p. 143). Nevertheless, the behaviour of the adopted insects, as far as I am aware, is not chemically regulated by the colony which adopts them, so the insects cannot be said to be directed by a master program.

There are, to be sure, some cases of symbiosis where the degree of co-operation is far closer: the host organism acquires new genes from the symbiont by a process of lateral transfer, and the symbiont becomes so integrated into the host that it is effectively a part of the host cell (an organelle). In such cases, a genuinely new organism emerges, but as the parts are spatially contiguous (one is subsumed within the other), it cannot technically be described as a superorganism, where the parts are physically separated.

The "unity" and "individuation" problems can now be addressed. The unity of the superorganism is guaranteed by its master program, which controls all the parts. Two superorganisms are individuated by their possession of different master programs. To return to the ant colony analogy: each colony has its own distinctive pheromones, by which it discriminates between nestmates and strangers. Within a colony, pheromones comprehensively regulate the activities of the colony: they are used to signal the presence of food, recruit help when an ant is in danger, identify other castes, inhibit the laying of eggs by the queen's daughters, and fix the percentage of larvae that grow up to be soldiers - all for the benefit of the colony (Holldobler and Wilson, 1994, p. 55).

The idea that a superorganismic animal could have parts that functioned in different locations may seem counter-intuitive, but Varner has suggested a helpful analogy:

[S]uppose that, instead of being connected to my brain by long networks of nerves, the muscles in my hands were operated by a kind of natural radio signal. Then I could detach my arms and go down the hall to check my mailbox without leaving off typing (assuming, of course, that I could remove my arms without bleeding to death and that I am a touch typist!) (1998, p. 75).

Varner proposes a very inclusive criterion for individuation in asexually reproducing organisms: he suggests that "clonal reproduction never results in more than one individual, it just results in the one individual having noncontiguous parts" (1998, p. 74). He cites the example of an aspen grove, whose trees, although they appear distinct, are actually connected by the roots, underground. The grove is certainly one tree, and it is fundamentally no different from a live oak that splits into several branches just above the ground (p. 75). Next, he argues that if we sever the connections between the roots below the ground, we still have one tree, and finally, he avers that even if we remove one of the severed "parts" and plant it far away, it makes no ontological difference. I find this example unpersuasive. For my part, I can see no reason to call the removed tree (as I prefer to call it) a part of a larger whole, if its behaviour is no longer regulated by the whole to which it formerly belonged, and if it can survive and thrive without the whole.

Another, related question is whether an organism needs to be temporally contiguous. Could there be an organism which lived for an interval of time, only to have its life stopped and subsequently re-started again when conditions were favourable? Once again, the answer appears to be affirmative. Close approximations can be found in the real world - for instance, bacterial spores, a resting phase displayed by some types of bacteria in response to adverse environmental conditions. Certainly, philosophical problems of identity would arise if the organism were to completely disintegrate and subsequently be re-constituted. In that case, it could be argued that it was no longer the same organism. However, if the information that constitutes the organism's "formal cause" is preserved within the "body" (or "matter") of the organism, even while its life-functions have ceased, then the issue of identity over time becomes fairly unproblematic. In Aristotelian terms, what seems to be required for identity is that the form be preserved over time within the organism's matter, even when every organismic function (ergon) has been switched off.

It remains to discuss whether any other attributes that have traditionally been used to identify living things are truly necessary conditions for something's being alive. It will be recalled that former "Science" editor-in-chief Daniel Koshland listed seven pillars of life, the first of which was examined in section (i). The other "pillars of life" that Koshland identifies are: (2) improvisation, or a way of changing its master program (achieved on Earth through mutation); (3) compartmentalisation (a surface membrane or skin, and for large organisms, a subdivision into cells, in order to preserve the ingredients required for chemical reactions at their required concentrations); (4) energy (which on Earth comes from the Sun or the Earth's internal heat), to keep living systems metabolising; (5) regeneration (this includes reproduction), to compensate for the wear and tear on a living system; (6) behavioural adaptability to environmental hazards; and (7) seclusion, or some way of preventing one set of chemical reactions from interfering with another, in a cell.

The necessity of Koshland's third pillar, compartmentalisation (the existence of an intrinsic boundary), for living systems has already been resolved in the affirmative. The fourth and fifth "pillars of life" listed by Koshland (2002, pp. 2215-2216) are energy (to keep living systems metabolising) and regeneration (including reproduction) to compensate for wear and tear on the system. These are thermodynamic requirements, which as Wolfram rightly points out, are certainly not sufficient to define living systems. Are they necessary conditions for life? Given that all living things possess some kind of functionality, Koshland's energy requirement - that a living thing must be a thermodynamically open system - seems indispensable. The physical necessity of regeneration (given the laws of thermodynamics) also seems inescapable, to counter what Koshland calls the thermodynamic losses of a metabolising system.

Although he classifies it as an aspect of regeneration, Koshland also considers reproduction to be a necessary feature of living systems. He is careful to point out that the capacity to reproduce cannot be considered a defining property of living individuals (for then, as he remarks, two rabbits - a male and a female - would be alive but one, by itself, would be dead), but argues that reproduction is a necessity for any kind of living system, for thermodynamic reasons. According to Koshland, reproduction is necessary to counter the accumulation of slight imperfections in the constant resynthesis of bodily constituents during an individual's lifetime (in other words, aging). Reproduction gives a living system the opportunity to start over.

Koshland's thermodynamic rationale for reproduction is less than compelling. Surely, it is physically possible that an open system could exist that was capable of regenerating itself almost perfectly, even over a long period of time - say, a million or a billion years. Such a system might exhibit the finality, form and functionality of a living system, even without the capacity to reproduce - and then, just die out. (Precisely how and whether such a system could arise in the first place is another matter.)

However, there are two other weighty reasons for considering reproduction seriously as a necessary feature of life. The first is the fact that every kind of living thing with which we are familiar, reproduces. As Aristotle remarks (De Anima 2.4, 415a28-29):

For this is the most natural of the functions of such living creatures as are complete and not mutilated and do not have spontaneous generation, namely to make another living thing like themselves, an animal an animal, a plant a plant ... (1986, p. 165).

Leaving aside Aristotle's essentialism and his quaint belief in spontaneous generation, his point remains valid. Reproduction is found in every species of living thing on earth. In Aristotelian terminology, we might say that an organism's capacity for reproduction is a "proper accident" of its soul - a necessary by-product of the more fundamental property of being alive. However, Aristotle (De Anima 2.4, 415a24-25) also classifies nutrition as a function of the "first and most general faculty of the soul, in virtue of which all creatures have life" (1986, p. 165). This invites the question: could there exist a life-form on another planet, which possessed the faculty of nutrition without reproduction?

The second reason for regarding the ability to reproduce as a hallmark of life is the Darwinian or historical paradigm of biology: living things cannot be considered apart from their genes, and the reason why living things have the genes that they possess is that those genes have out-competed other genes in a four billion-year evolutionary race to replicate themselves. According to this paradigm, reproduction could be considered as the defining characteristic of life. However, there are two major reasons for treating the Darwinian argument with caution. First, we know of only one planet so far which supports life, so it seems unwise to generalise about life-forms on other planets. Second, if we allow reproduction to define life, then we have to admit a host of systems, including abstract computational systems, that "bear no other resemblance to ordinary living systems" (Wolfram, 2002, p. 824).

Even if we cannot go so far as to define life simply in terms of reproduction, we cannot dispense with it as a necessary condition until we discover some mechanism whereby a life-form could originate on a planet, without some process of self-replication being involved. (Interestingly, a new theory described in "Nature" magazine (4 December 2002) suggests that cells, whose walls were originally made of iron sulphide deposited by hot springs, originated first and served as incubators for organic molecules, which eventually acquired the ability to self-replicate. In other words, the cell may be a more basic feature of life than reproduction.)

Koshland's second pillar, improvisation, or a way in which an organism can change its master program (achieved on Earth through mutation), is a necessary condition for life if the ability to evolve is taken as fundamental to life (i.e. if we view life according to the Darwinian paradigm). Because we are familiar with life on only one planet, the universality of this paradigm has to remain a philosophically open question.

Koshland's sixth pillar (behavioural adaptability to environmental hazards) seems unexceptionable, but rather vague. Koshland elaborates:

behavioral manifestations of adaptability are a development of feedback and feedforward responses at the molecular level and are responses of living systems that allow survival in quickly changing environments (2002, p. 2216).

The obvious qualification is that adaptability, like rapidity of environmental change, is a matter of degree. Some organisms live in very stable environments; others, like ourselves, are in a continual process of adjustment.

Koshland's seventh pillar, seclusion, or some way of preventing one set of chemical reactions from interfering with another, within a cell, is guaranteed in terrestrial organisms by specific enzymes that work only on the molecules for which they were designed. One might think that only an organism that was based on specific chemicals would have such a need, but Koshland offers an analogy, which could apply to hypothetical organisms (discussed above) whose parts communicated by electromagnetic rather than chemical signals. He likens seclusion to insulating an electrically conducting wire so that it is not short-circuited by contact with another wire. The gist of the argument seems to be that organisms, having a large number of parts and an even larger number of interactions, require a certain degree of regulation to ensure that the interactions work properly. The workings of this internal regulation could be explained by considering the formal properties of the organism.

So far, the only terrestrial organisms we have examined have been cellular life-forms. Opinion is divided on whether acellular life-forms such as viruses should be considered to be alive. I discuss this question at further length in the Appendix (outside the "thesis proper"). I conclude that despite the inadequate rationale ("They reproduce") offered by most people who believe that viruses are true life forms, there are sound Aristotelian reasons for viewing them as bona fide organisms. They appear to satisfy the formal, final and material requirements for being alive, as well as instantiating most of Koshland's "seven pillars of life". I also argue that although a virus is a life-form, it is only alive in a secondary sense of the word: it participates in the life of its host, to use a Platonic metaphor.

Appendix: Are viruses alive?

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