Recently, that is since 1975 or so, the view has arisen that a living thing is something that satisfies several conditions.
In 1966 George C. Williams introduced the notion of an “evolutionary gene” in his Adaptation and Natural Selection, which was, he said, a “cybernetic abstraction”. This idea was taken up by Richard Dawkins in his The Selfish Gene.
Dawkins posited that evolution had some necessary and sufficient criteria:
There had to be replicators with the following properties:
- Longevity (over evolutionary time)
- Fecundity (more made than can survive)
- Fidelity (nearly perfect copying)
and these had to be contained in vehicles, which were the bodies that they “made”. David Hull, a philosopher, revised this to interactors, which were ecologically active agents, that determined the fitness of the replicators over evolutionary time.
The Hull-Dawkins Distinction, as it is now known, is considered by many to be the foundation for a “Universal Darwinism“, a generalised account that applies to any evolutionary process. Dawkins himself applied it to cultural evolution, and Hull has published with an immunologist and an ethologist a general theory of selection (Hull et al 2001).
If things evolve, then, they have these properties, and if they have these properties, then they are Darwinian things, and hence are prima facie alive. On a Universal Darwinian view, if it evolves, then it has the properties that are sufficient for life. So cultural ideas are like “mind viruses” that “get” themselves replicated by parasitising our brains.
Problems with these criteria are obvious as a definition of “life”: ideas are obviously not alive. They do not have the properties of living systems like metabolism. Nor do Animats and genetic algorithms.
So we are left at the end with a series of problems.
- If we include overly general criteria, we include nonliving things
- If we restrict our criteria the way Orgel does, we exclude non-terrestrial life
- If we try to generalise the physical properties of terrestrial life in a way that will include non-terrestrial biology, we run the risk of falling into one or two of the previous problems.
So, how to proceed? Can we define “life”? It depends a lot on how we think general terms get their meaning. Traditional philosophy thought, following Aristotle’s example, that if we only defined them clearly and unambiguously enough, we could include what we wanted to include and exclude what we wanted to exclude. This may be the problem.
Language in science gains its meanings in all kinds of ways. One is for a term to derive its meaning from a theoretical context. Hence “mass” is defined in one way in Newtonian contexts, and a different way in Einsteinian. But biological terms are usually applied first to exemplars, prototypical examples that we are familiar with, and then applied increasingly generally until they break down.
This explains why so many biological terms, that are not theoretical, break down at the periphery. We devise terms for things that are unproblematic and then expand the domain over which they apply until sufficient difficulty arises. One of the obvious features of our experiential world is that things live. So we take exemplary cases and try to expand the use of the term until we get to viruses, prions, and eventually extraterrestrial biology. The problem is that we are using analogy, not homology.
All life on earth, barring Carol Cleland’s possible but as yet undetected parallel biologies, evolved from a single origin of life event, and subsequent elaboration and reticulation of the lineages that evolved from it. So to define “Earth Life”, just point at the two most diverse examples one can find, and say “life is anything that evolved from the last common ancestors of those things”. This is definition by evolutionary homology, ostensively anchored in the world by pointing.
But analogous definition is “something that is like that” where “that” is some clear case that you point at. This is what motivated Aristotle. The trouble is that, as we have seen, “that-ness” is something too vague or inclusive to clearly identify what it is we are talking about (unless we restrict the “thats” to an evolutionary group such as metazoans, or fungi, or plants, etc.).
Analogy is not a good way to proceed in scientific definition, because it is subjective. It depends not on the objective properties of the things defined, but on the criteria that the researcher thinks is significant. In short, it defines facts about scientists, not the subjects of study. The use of theoretical definitions, however, attempts to avoid this because theories are supposed to be rigorous descriptions of objective reality. Hence definitions of “electron” work, because the theory has been refined to the point where it is entirely adequate for the domain of physics.
In biology, however, most definitions must necessarily be ostensive and phenomenally based, unless we apply a phylogenetic definition. This means, though, that an ostensive definition based on common ancestry excludes life on other planets. And now it is time, perhaps, to qualify what it is that is being defined: the last common ancestor of our two most unrelated organisms defines, extensively, only “Life on Earth”. It doesn’t define “life” universally. If we find that Jupiter’s clouds do enough of the things that we ordinarily count as living by analogy (reproduction, ecological interaction), then we will apply our pre-existing word “life” to them, and thereafter have to use an extensional definition for “Jovian Life”, and so on.
Will there be a general theory of life? I doubt it. Or rather, I think we already have that general theory – it’s the laws of chemistry, thermodynamics, and physics in general. But knowing all about physics won’t tell us what sorts of particular tricks the physical world has for doing complex stuff like reproducing. To know that, we have to encounter it first, and then work out how it happened according to the rules of What Chemistry Can Do. And Physics. And Systems. And so on…
Hull, D. L., R. E. Langman, and S. S. Glenn. 2001. A general account of selection: biology, immunology, and behavior. Behav Brain Sci 24 (3):511-28; discussion 528-73.