Natural Classification

The principle upon which I understand the Natural System of Botany to be founded is, that the affinities of plants may be determined by a consideration of all the points of resemblance between their various parts, properties, and qualities; that thence an arrangement may be deduced in which those species will be placed next each other which have the greatest degree of relationship; and that consequently the quality or structure of an imperfectly known plant may be determined by those of another which is well known. Hence arises its superiority over arbitrary or artificial systems, such as that of Linnaeus, in which there is no combination of ideas, but which are mere collections of isolated facts, not having any distinct relation to each other.

This is the only intelligible meaning that can be attached to the term, Natural System, of which Nature herself, who creates species only, knows nothing. Our genera, orders, classes, and the like, are mere contrivances to facilitate the arrangement of our ideas with regard to species. A genus, order, or class, is therefore called natural, not because it exists in Nature, but because it comprehends species naturally resembling each other more than they resemble any thing else.
The advantages of such a system, in applying Botany to useful purposes, are immense, especially to medical men, with whole profession the science has always been identified. A knowledge of the properties of one plant is a guide to the practitioner, which enables him to substitute with confidence some other that is naturally allied to it; and physicians, on foreign stations, may direct their inquiries, not empirically, but upon fixed principles, into the qualities of the medicinal plants which nature has provided in every region for the alleviation of the maladies peculiar to it. To horticulturists it is not less important: the propagation or cultivation of one plant is frequently applicable to all its kindred; the habits of one species in an order will often be those of the rest; many a gardener might have escaped the pain of a poisoned limb, had he been acquainted with Natural affinity; and, finally, the phenomena of grafting, that curious operation, which is one of the grand features of distinction between the animal and vegetable kingdoms, and the success of which is wholly controlled by ties of blood, can only be understood by the student of the Natural System. [John Lindley, A Natural System of Botany, Longman et al. 1836, viii]

This is almost exactly the justification of phylogenetic analyses, contrary to those who think that there is something else important about them, like history or technique.

After Linnaeus had settled on the older mechanism of hybridisation of genera with other genera or with varieties formed by geographical conditions as the cause of new species, the topic began to pick up speed. Hybridisation remained the usual method as late as the 1830s (e.g., in Lindley) but two developments were crucial in the rise of the “species problem” of the nineteenth century.

One was to do with the degree of freedom in hybrids. Linnaeus had originally held that all species that could exist did, and only later in his work, after around 1751, did he allow for there to be as-yet unfilled possibilities. He had held that all species formed a continuum, like countries with adjacent borders. In his later work, he permitted some regions of the “species landscape” to be unfilled, which hybrids were able to enter. As Peter F. Stevens (1994) has shown, with Antoine Laurent Jussieu (1748–1836) in the French tradition, these regions became more sparsely inhabited. Mechanisms for new species in the uninhabited regions were called for. A “natural system” – one in which the relations were objective (“in nature”) – was called for. But most, such as Adanson and Jaume Saint-Hilaire (1772–1845), still believed that when we had found all species there would be no gaps (Stevens 1994:95ff).

Pierre Maupertuis

Pierre Maupertuis

The other, however, was the introduction of evolutionary, or transmutationist, ideas. Linaneus’ Systema Naturae introduced the notion of species fixity when it was first published in 1735. A mere decade later, the first transmutationist notion of species was published by Pierre Maupertuis, a physicist who also discovered something like the Mendelian ratio. In his Venus Physique (translated as The Earthly Venus) in 1745, Essai de cosmologie, in 1750, Maupertuis wrote:

Could one not say that, in the fortuitous combinations of the productions of nature, as there must be some characterized by a certain relation of fitness which are able to subsist, it is not to be wondered at that this fitness is present in all the species that are currently in existence? Chance, one would say, produced an innumerable multitude of individuals; a small number found themselves constructed in such a manner that the parts of the animal were able to satisfy its needs; in another infinitely greater number, there was neither fitness nor order: all of these latter have perished. Animals lacking a mouth could not live; others lacking reproductive organs could not perpetuate themselves… The species we see today are but the smallest part of what blind destiny has produced… [translation from Glass 1959:58]

In another work he wrote in 1751:

Could we not explain in this manner [of fortuitous changes] how the multiplication of the most dissimilar species could have sprung from just two individuals? They would owe their origin to some fortuitous productions in which the elementary parts [of heredity] deviated from the order maintained in the parents. Each degree of error would have created a new species, and as a result of repeated deviations the infinite diversity of animals that we see today would have come about. [Systèm de la Nature 2:164, quoted in Terrell 2002:338]

Maupertuis’ idea is a revised version of Empedocles’ lucky monsters theory, but it differs in that the parts which recombine are heritable variations of traits caused by blind chance based on what prior parental traits were inherited with “error”. There is no real idea of population or competition, but it is at least in the direction of natural selection.

Later evolutionary (or better, transformist) views included limited varieties such as that of Buffon, through to the universal transformism of Erasmus Darwin or Lamarck. One author was Denis Diderot, whose theory, or better passing comment, is this:

It seems that nature has taken pleasure in varying the same mechanism in a thousand different ways. She never abandons any class of her creations before she has multiplied the individuals of it in as many different forms as possible. When one looks out upon the animal kingdom and notes how, among the quadrupeds, all have functions and parts—especially the internal parts—entirely similar to those of another quadruped, would not any one readily believe (ne croirait-on pas volontiers) that there was never but one original animal, prototype of all animals, of which Nature has merely lengthened or shortened, transformed, multiplied or obliterated, certain organs? Imagine the fingers of the hand united and the substance of the nails so abundant that, spreading out and swelling, it envelops the whole and in place of the human hand you have the foot of a horse. When one sees how the successive metamorphoses of the envelope of the prototype—whatever it may have been—proceed by insensible degrees through one kingdom of Nature after another, and people the confines of the two kingdoms (if it is permissible to speak of confines where there is no real division)—and people, I say, the confines of the two kingdoms with beings of an uncertain and ambiguous character, stripped in large part of the forms, qualities and functions of the one and invested with the forms, qualities and functions of the other—who then would not feel himself impelled to the belief that there has been but a single first being, prototype of all beings? But whether this philosophic conjecture be admitted as true with Doctor Baumann [Maupertuis*], or rejected as false with M. de Buffon, it can not be denied that we must needs embrace it (on ne niera pas qu il faille I’embrasser) as a hypothesis essential to the progress of experimental science, to that of a rational philosophy, to the discovery and to the explanation of the phenomena of organic life. [Denis Diderot, 1753, Pensées sur l’interpretation de la nature, ch. XII, as translated by Lovejoy 1904: 325]

In other words, things evolve over time, but there are no divisions between them, so any classification into species is arbitrary. Hence there can be no “origin of species”, unlike Maupertuis’ views. Diderot here represents the views of the Encyclopedists, of whom he was a leading member as the chief editor of the Encyclopédie. A similar view was espoused earlier by Charles Bonnet (1720–1793), who arrayed species along a continuum from simplest to most complex. Bonnet’s vision was, however, static rather than transformist. Reaction to his scheme was mixed. One rejection was by Peter Simon Pallas, in Elenchus Zoophytorum (1766) , where he argued that we had to represent living things as a branching tree. Again, Pallas was no transformist, but he laid the ground for something like tree thinking, which became the problem Darwin tackled.

Maupertuis’ idea of fitness being what remains after chance has caused variations was not regarded as terribly likely, and on the point of atheism by many. Apart from the hybrid theory, and this, however, there was another idea, not unlike Linnaeus’. That is, speciation by geographical variation. The key figure here is Buffon.


Statue Buffon Carlus


Georges Louis Leclerc, comte de Buffon (1707–1788) was an influential figure in the eighteenth century. He tended the King’s Garden (Jardin du Roi, later the Garden of Plants, Jardin des Plantes) and wrote a multivolume series entitled Natural History (Histoire Naturelle 1749–1788, in 36 volumes) in which he appeared to post-Darwin scholars to be given an evolutionary theory. In an early volume (1751: “The Ass”) he wrote that species today are “nearly” what they were when created, and that fertility between individuals is what makes a species:

… we can draw a line of separation between two species, that is, between two successions of individuals who reproduce, but cannot mix; and, as we can also unite into one species two successions of individuals who reproduce by mixing.  This is the most fixed and determined point in the history of nature. All other similarities and differences which can be found in the comparison of beings, are neither so real nor so constant.

Later, however, he seemed to present a transformist view: species were not what Linnaeus called species but more like what came to be known as “families”. All canine species formed a group that degenerated from the original dog, all feline species likewise, along with horses, sheep, and cattle. The causes of this degeneration were local climatic and geographical influences upon the seminal fluid and its expression. In the initial essay, he argued that the Ass was not a degenerate horse, because it was infertile when crossed with horses. Later, in “Of the degeneration of animals” (1766), he called the ass a degenerate form of the horse, along with the zebra. As Farber notes (1975: 69):

What Buffon envisioned was the production, in time, of natural genera created by the gradual modification of species. For each genus or family (Buffon used the two words interchangeably) there was an original premier souche [primary stock] that had degenerated into several recognizably different varieties, what we commonly call species. One could show that all the individuals came from a common ancestor by noting their morphological similarity and their alleged ability to interbreed.

This was due to an interior mould (moule intérieur) that caused the embryo to develop into the adult organism; under different conditions, that mould would be expressed differently. Thus a genus or family was the “true” species, and our “species” but varieties of the primary stock. We can classify them all as related because they are the constant forms, despite their outward differences. In modern terms we would call this developmental systems modified by epigenetics (but note: epigenetics means something very different in the period under consideration than it does today. Here it means that the embryo is formed through a developmental process rather than being preformed in the seminal fluid).

Buffon, being a Lockean empiricist as well as a Lockean nominalist, even did a number of cross breeding experiments, with some success, to prove his theory of degeneration. While he got little result from crossing pigs and peccaries, dogs and wolves, dogs and foxes, or hares and rabbits, he did get results from goats and sheep (Roger 1997: 319). Unlike Linnaeus, his vision of hybridisation between “species” in the Linnaean sense was regressive rather than the origin of new species.

Next, I shall discuss the eighteenth century evolutionists Erasmus Darwin and Lamarck.


Farber, Paul Lawrence. 1975. “Buffon and Daubenton: Divergent traditions within the Histoire naturelle.” Isis 66 (1):63-74.

Glass, Bentley. 1959. “Maupertuis, pioneer of genetics and evolution.” In Forerunners of Darwin 1745-1859, edited by Bentley Glass, Oswei Temkin and William L. Straus, 51-83. Baltimore: Johns Hopkins.

Lovejoy, Arthur O. 1904. “Some eighteenth century evolutionists. II.” Popular Science Monthly LXV (August):323–340.

Roger, Jacques. 1997. Buffon: A life in natural history. Translated by Sarah Lucille Bonnefoi. Edited by L Pearce Williams, Cornell History of Science Series. Ithaca, NY: Cornell University Press.

Stevens, Peter F. 1994. The development of biological systematics: Antoine-Laurent de Jussieu, nature, and the natural system. New York: Columbia University Press.

Terrall, Mary. 2002. The man who flattened the earth: Maupertuis and the sciences in the enlightenment. Chicago: The University of Chicago Press.


*. Maupertuis initially published Système de la Nature, based on his PhD dissertation, under the pseudonym of Dr Baumann.


Carolus Linnaeus

One of the fundamental aspects of evolution is speciation. This is the process by which more species come into being, and there are many different definitions and mechanisms that have been proposed by biologists in the last couple of centuries. I aim to write an occasional series on what it is supposed to be at various times in the history of biology, as well as the theoretical and, if I get to it, professional aspects.

For there to be speciation, however, there needs to be the possibility of new species. The common view is that this requires a theory of evolution, but in fact, biologists from Linnaeus onwards have posited the generation of new species, even in the absence of anything resembling evolution. For example, during the middle ages, it was commonplace to think new species arose by spontaneous generation (that was the main method that writers about the Ark proposed, along with hybridisation). As I have argued, the notion of “species” itself arose from consideration of the Ark story, as more and more species were reported by travellers and colonisers.

Linnaeus was a creationist, as were nearly all naturalists during the 18th century. He held that varieties within species, and possibly even some species themselves, were but local forms caused by the action of soil, climate and weather. However, he allowed, later in his life, for a kind of speciation by hybridisation. First of all is the famous comment of the species Thalictrum lucidum:

Is the plant sufficiently distinct from T. flavum? It seems to me a daughter of time. [Species plantarum]

What he meant by this is unclear. It is not enough to base a speciation theory on. But he then described four species of Scorpiurus and says

It is beyond all doubt, that all these formerly arose from a single species, and the alteration in the environment is not sufficient for their creation: what commingling has then given rise to the constant plants?

He repeats this about species of GeraniumCalendulaSonchus, and Campanula and the suggestion is they formed by hybridism. As Ramsbottom (1938) from whom I get this, says:

Five varieties of Solanum nigrum appear to be the offspring of hybrids. … he states that the varieties between Fumaria spicata and F. capreolata, judging from their flowers, might be considered as F. oficinalis and queries whether they are hybrids.

Perhaps equally striking is the treatment of varieties in ‘Species Plantarum’ when we bear in mind the definitions repeated two years previously. Far from being merely variations in non-essential characters, they are treated in the same way as species,and as may be seen from some of the quotations already given it is sometimes queried whether what is described as a species is only a variety or vice versa.

Rowbottom doesn’t think Linnaeus has changed his mind from the earlier Philosophica botanica. Instead he thinks this is something Linnaeus had always allowed. Linnaeus’ student Daniel Rudberg in 1744 had discussed the possibility of hybrids forming. And in 1746, another student, Johannes Gustavus Wahlbom, had discussed hybridisation in tulips. He explained it as degeneration: related species were a degradation of the original species, a view Rowbottom ascribes to Aristotle’s student Theophrastus. A modern botanist would assign this to plesiomorphic (underived) developmental systems, which is not so far removed. In  1751, his student Johannes J. Haartman described a hundred species thought to be hybrids on taxonomic grounds. Several other students made similar comments, quoted by Ramsbottom.

Although Linnaeus famously supposed that a genus, Peloria, was the result of hybridism between a flower of Linaria and some unknown plant, which he published in 1744 after Gmelin had responded to a letter from Linnaeus with news that he had found some hybrids too (Gardiner 2001) [1], he finally made his views explicit in a tract, Disquisition on the sex of plants, in 1760, in which he wrote:

There can be no doubt that these are all new species produced by hybrid generation. And hence we learn, that a mule offspring is the exact image of its mother in its medullary substance, internal nature, or fructification, but resembles its father in leaves. This is a foundation upon which naturalists may build much. For it seems probable that many plants, which now appear different species of the same genus, may in the beginning have been but one plant, having arisen merely from hybrid generation. … these Geraniums, I say, would almost induce a botanist to believe, that the species of one genus in vegetables are only so many different plants as there have been different associations with the flowers of one species, and consequently a genus is nothing else than a number of plants sprung from the same mother by different fathers. But whether all these species be the offspring of time; whether, in the beginning of all things, the Creator limited the number of future species, I dare not presume to determine. I am, however, convinced, this mode of multiplying plants does not interfere with the system or general scheme of nature

So Linnaeus held that from an initial plant with a variety of possible forms and parts, hybrids could generate some, but not an open-ended number, of new species. In a tract published two years after this, his student Johannes Mart. Gråberg wrote:

We imagine that the Creator at the actual time of creation made only one single species for each natural order of plants, this species being different in habit and fructification from all the rest. That he made these mutually fertile, whence out of their progeny, fructification having been somewhat changed, Genera of natural classes have arisen as many in number as the different parents, and since this is not carried further, we regard this also as having been done by His Omnipotent hand directly in the beginning; thus all Genera were primeval and consisted of a single Species. That as many Genera having arisen as there were individuals in the beginning, these plants in course of time become fertilized by others of different sort and thus arose Species until so many were produced as now exist. … That also some Genera multiplied into very numerous Species…. That these Species were sometimes fertilized out of congeners, that is other Species of the same Genus, whence have arisen Varieties.

Todays genera are the original creations of God. Ramsbottom says

The same theory of progress from simple to compound, from few to many (e simplice progressus ad composita; e paucis ad plura!) was repeated in the sixth edition of ‘Genera Plantarum’, 1764.

Linnaeus fixism was widely adopted, although it was in part based upon an artificial system, by Linnaeus’ own admission. He wanted a natural system – one that explained the underlying causal relationships between plants – but never was able to produce it. As late as 1830, John Lindley was calling Linnaeus’ system “natural”, remarking

Nature herself, who creates species only (Lindley 1830, xvi).

The genera were God’s creation here, too. Linnaeus’ ideas that species were generated was a commonplace. His ideas of the diversification of species by hybridism, however, while it was not used as the foundation for much research, became part of the botanist’s mental toolkit. This is not surprising, though, as naturalists had used hybridism as an explanation of new and deviant species since Aristotle had written about it for animals in the Historia Animalium, and Theophrastus in his Enquiry into plants. Later, hybridism was the foundation of Mendel’s researches, to which we shall return.


Gardiner, Brian G. 2001. “Linneaus’ species concept and his views on evolution.” The Linnean 17 (1):24–36.

Lindley, John. 1830. An introduction to the natural system of botany: or, A systematic view of the organisation, natural affinities, and geographical distribution, of the whole vegetable kingdom: together with the uses of the most important species in medicine, the arts, and rural or domestic economy. London: Longman, Rees, Orme, Brown, and Green.

Ramsbottom, John. 1938. “Linnaeus and the species concept.” Proceedings of the Linnean Society of London 150 (192-220)


1. It was not. It is an epigenetic mutation, neither concept of which was available to Linnaeus.

Despite marking scores of essays, after having taught a subject intensive, and preparing various papers, I get to review some books. This means reading them, familiarising myself with the technical literature, and so on. So I thought I’d do a brief summary of them for you now:

The first is this:


M. A. Khalidi, Natural Categories and Human Kinds: Classification in the Natural and Social Sciences

This is an excellent summary and discussion of the idea of natural kinds in science (and as such makes a nice companion piece to my book). Where I and my coauthor Malte Ebach approached the philosophy of classification from the perspective of the sciences, Khalidi approaches the sciences from the perspective of the natural kinds debate in philosophy. He discusses some issues, such as whether classes can “crosscut” other classes, what the status of kinds are in science and philosophy, and essentialism (which has been revived lately in the philosophy of science). In particular he rejects essentialist accounts of kinds because of their problem in inductive projectibility.

So far there are a few slips: he conflates Linnaean taxonomy with phylogeny, for instance. But it is a good book. I recommend it. The review will appear in Notre Dame Philosophical Reviews.


God in the Age of Science?: A Critique of Religious Reason by Herman Philipse

Philipse addresses the standard arguments, and some not so standard recent arguments, for the rationality of belief in God. He specifically addresses Richard Swinburne’s arguments, and concludes that the most promising is a Bayesian account Swinburne proposes. He discusses reformed theology, which is often ignored by philosophers. Being a professor in a Dutch university (Utrecht), he is well placed to consider many of the theological arguments they present. As many of these underpin the evangelical apologetics found in the United States, this is significant. But he also spends considerable time dealing with the usual arguments for God’s existence, such as the cosmological argument that depends upon modal logics, as well as natural theological arguments.

Godfrey Smith

Philosophy of Biology (Princeton Foundations of Contemporary Philosophy) by Peter Godfrey-Smith

PGS, as he is known in the discipline, is perhaps the leading philosopher of biology today. One expects, then, that this will be a great book, and so it seems given a quick read. It looks like the lecture notes of a course, which is a good thing, since it presents an introduction to a complex field. I have quibbles, of course, but none of them are particularly telling. I would use this as a textbook in a course, as it offers a good summary of the issues and links to literature that the student can use to explained her knowledge.

Griffiths and Stotz

Genetics and Philosophy: An Introduction (Cambridge Introductions to Philosophy and Biology) by Paul Griffiths and Karola Stotz

This is an excellent and nuanced introduction to the philosophy and biology of genetics, including discussions of the idea of “gene”. They introduce a notion of genetic information – “Crick information” – which is basically the structural specificity of genes and their products. I recommend this book unreservedly, and will be discussing it in more detail later.

Nature of classification

It occurs to me that I haven’t plugged my own book here. What a failure on my part! It was published in December, so it is really time I did so.

In this book, Malte Ebach and I discuss a topic not often discussed in the philosophy of science: the classification of nature in the absence of a theory that delineates natural kinds. Since when we begin the investigation of a new field, there is no theory of that domain yet, by definition, how do we begin? The standard answer has been that we take the kinds posited by a closely related theory and refine the theory to include that domain. But this doesn’t account for the domains that have no closely related theories that could potentially cover them. For example, classifications of living things began, and were relatively sophisticated, well in advance of anything resembling a theory of life. Instead, researchers refined the folk taxonomies in existence, and approached the domain in a naive empirical fashion. In short, they looked for patterns in the data.

The Standard View is that what we observe is determined by our theories – we literally cannot see what we do not expect. This, the theory-dependence of observation hypothesis, presumes rather bluntly that the observational salience of phenomena is first constructed and then observed. This may be true in cases where there already exists an elaborated theory that is relatively relevant to this new domain. And it is almost dogma that we do not have what Michela Massimi has called “ready-made phenomena”. It is this we take some guarded exception to. There is the following conundrum: if we cannot see until we have a theory, and yet we can learn to see phenomena as children, then “theory” must include not only the formal explanatory models of science, but indeed any disposition to see some things and not others, which makes theory everything we are biased to observe. This is, I think, to attenuate the notion of “theory” so far that it becomes a meaningless term. We see much of what we see because we evolved to see it, so our evolutionary past becomes theory. A concept like that cannot be useful in science. It is much better to separate our dispositions inherited from biology and culture out from the technical apparatus of theory in science, so we capitalise the latter: Theory.

Our brains are wired to find patterns in data, in large part because they are neural networks, and that is what neural networks are good at. We are classifier systems. A classifier system finds regularities in large data sets (including, but not just, measurements using instruments like a pan balance or a thermometer), and once they are found, they call for an explanation. Now, I realise that induction is supposed to be a problem in philosophy, as no finite number of observations of this kind can determine a unique general solution or generalisation, and yet, that is what every child who learns a language or not to touch hot things does. There simply are some ready-made phenomena, even if we cannot justify the regularities deductively. I think we might do so abductively, though, just as Peirce thought.

So, we formulate our classifications and find patterns to explain. We give examples of this in meteorology, pedology (soil science), chemistry, psychiatry, and several other cases. But of course the other kind of natural kinds occur too. It actually is the case that some kinds are formulated by our theories. How do we relate the two? The answer lies, we argue, in the dynamic nature of science. Science is not just a theory-driven enterprise, but when we have theory, we test it and refine it on empirical foundations. If a theory asserts the existence of some kind, and we find that it does so with precision and accuracy, then we have confidence that the kind is real. But if we find the kind is not matching the patterns we identify from direct observation, whether experimental or field observations, then there is something wrong with the theory (i mean here, all the theories used in the investigation of the domain; some theoretical foundations include distal theories as components).

We argue that theory-naive (or just “naive”) classifications are formed by a process of trial and error, to determine the marks that make the classification stable and useful. We call these marks, following the biological practice, homologies, and the marks that are not good discriminata, analogies. In brief, homologies allow projection of our inductive conclusions, while analogies offer no more information than is used in their construction. To give an example, all homologically related organisms in a classification will tend to share the same sets of marks (“characters”), so if most or all of that group have been found to have a mark, any newly discovered kind of organism will very likely have it too, even if that has not been observed. Homological classifications are, as Goodman termed it, projectible.

This isn’t a general solution to the grue problem Goodman formulated, but then classification isn’t about induction. It is about recognising patterns and using those patterns to refine our beliefs. We have to do this in order to survive, as Quine noted a long time ago, but at best a classification is something that is highly defeasible, and the relation of classification to Theory is itself dynamic. A classification is at its best, the beginnings of knowledge. It is a call for Theory to explain what we see.

In one chapter – Monster and misclassifications – we discuss how unnatural classifications are formed and what they tell us. I would summarise it this way: a natural classification formed on homologies, which are usually causal regularities or etiological classifications, tells us two things. It tells us what we, the observers, find salient (as does any statement about the world, for there are an infinite number of things we might say), but it also tells us about the structure of the domain being observed. It is a two-place system, us and it. But a monstrous classification (which includes paraphyletic classifications in biology, for example) tells us only about our salience dispositions. While there are observations being made of the world, the selection of the marks themselves is all about us, and so a monstrous classification is a statement about us and our dispositions only.

Finally, in order to evade the political aspects of classification, we propose a neutral, functional, set of terms to discuss what scientists mean by classificatory terms. Science is done by people, and people, among other things, play political games (in the Wittgensteinian sense of “game”) in order to mark in-group from out-group loyalties. This, while it may seem to be less than admirable from a purely formal perspective, is an irreducible aspect of human science, and indeed it may tend to drive science by motivating, as David Hull called it, the “I’m gonna get that son of a bitch” responses to claims, thus acting as a selective pressure against groupthink. Since all scientists do or ought to take empirical evidence as their starting point, if an opponent can show that a model is not empirically adequate enough, even abstract ideas like “monophyly” in biology can be revised.

I commend our book to my readers. It seems like a silly thing to write about, but I believe the classification of nature has for too long been seen as a merely conventional practice in science. Nobody would deny that there is a conventional aspect to science, just as there is any other human social activity, but it is time to abandon the idea that this is all there is to it.

One final note. This book was written, at the editor’s instructions, to be accessible to both philosophers and scientists, so the language may seem a little untechnical. The result is surely that the scientists will find it too difficult to read, and the philosophers not difficult enough. I beg of my readers not to think that because of this it is either useless or shallow.


Life, I believe, is what physics does on one particular planet on a Wednesday. More exactly, it is a series of chemical and physical dynamics that occurs between 3.85 billion years ago and now on this planet.

Ferris Jabr, an editor at the Scientific American site, has a piece entitled “Why Life Does Not Really Exist”, and in it he challenges the NASA definition of life as any system that is capable of undergoing a Darwinian process. He relies upon the work of Carol Cleland, who worked with the NASA Astrobiology program before it was shut down. He and Cleland deny that there is anything that applies to all and only those processes we call “life”. This, he thinks, sinks the Astrobiology project that seeks to find life elsewhere in the universe, because they lack any clear criteria of what they are looking for.

Now I have dealt with the definitions of life before here: The Meaning of “Life”, “What is ‘Life” Again?” and “What is ‘Life’ At Last?”. In it I made a point that the meaning of words like “life” tend to be prototypical. You pick something that is unquestionably alive, point to it, and say “life” is like that thing there. This is both an ostensive definition (you anchor it to the pointed thing) and an extensive definition (you list the things it applies to, in this case through some sort of resemblance or similarity metric). What it is not, though, is a theoretical definition. Scientists seem to like seeking theoretical definitions of key terms. They go to great lengths and have long discussions about what the “true” definition is of terms that are general, useful, and yet elusive. There are several ways to go. One is to take some privileged theory and say that term X refers to all objects that play this role in the theory. This inevitably requires excluding some things that were included in the posse use of the term X. For example, when we discovered DNA, life was held to be that which has DNA, until we realised that some objects (some viruses) use DNA and are not alive by most understandings, and that there are things which do not use DNA, but use RNA. The prior scope of the term “life” overrode these theoretical definitions, although the battle was bloody and many careers died during it.

Another way to go is to seek a “general theory” of the term X, which means taking disparate properties and attempting to come up with a global model in which these things all have theoretical roles. The trouble with this approach is that the more you include, the more abstract the theory, like the Darwinian account. If life is what evolves, and chemical and computer programs can evolve, then you end up explaining almost everything at the tradeoff of having very little to say about the actual things. What makes a virus fitter than another is its physical ability to infect host cells and engage the cell’s reproductive machinery, not the having of a fitness of 0.7. Such theories, including systems theory, chaos theory, complexity theory and the latest of these trendy approaches, network theory, tend to be ways to anticipate what will happen, yes, but only after you have given the model a physical interpretation: mapping (for example) genes to replicators or cell surface molecules to fitness challenges. In effect these models tell you about a class of things the model tells you about.

So trying to come up with a theoretical definition of such terms (and the one I know most intimately, is, of course “species”) is in general a losing bet. We might recast this definitional question thus: “Is ‘life’ a natural kind?” This is what it implies: that in order for there to be meaning, scientific meaning, to the term “life”, it must be a kind of things in nature that are objective and so mandated by a best theory (if and when we get one). Anything else is subjective or “folk science” and should be abandoned. To some, by which of course I mean me, this is too harsh and thin a conception of scientific meaning. If Cleland is right, then there is no science of biology, never mind astrobiology.

David Hull once said that the only law in biology (and hence the only natural kinds we will find) is that all laws in biology admit of exceptions. Including, it appears, that one. Hull’s law is tied into a debate about kinds in science, and if there are no exception less laws in biology, we might be forced to say that the science, and hence the funding and credit given to biologists, is to be abandoned. At the very least this will cause ructions in university management. And yet, this is the century of biological science. How can we reconcile these conflicting issues? I believe it has to do with the role of phenomena, not theory.

Science does not enter into an enquiry fully armed with theoretical categories. Any new science (and biology was a new science 300 years ago) must begin by noticing something interesting. Usually, this is something that is interesting in the wider, nonscientific, culture. “Planets” were once wandering stars. Now they are largish balls of matter that orbit a “star” (now thought not to be a point of light on crystal spheres, but a very large ball of fusioning gas). In the process of noticing and then explaining these objects, once just folk objects, we have come to a rich understanding of things even if there are “failed stars”, and “dead stars” that were never once part of the popular cultural imagination.

Few phenomena in our world attract our attention as much as living phenomena. The reason for that is pretty obvious: if we didn’t attend to them well enough, we’d be dead. From that folk conception of animate and edible growing things, we developed our present taxonomies and explanations, but as we did so the comfortable certainties of life, many taken from Aristotle, have evaporated or been constricted to a small class of things. This is progress. Where once a biologist might have thought that what applies to E. coli applies to an elephant, we now know this is not true. They have, for a start, different ways of “reading” the genetic code. But nobody would then say an elephant is alive and an E. coli is not. “Life” is a polymorphic property. We have many things we think are alive although they do not share the same properties, at least at a physical level.

So “life” is not a natural kind, if by that we mean it is not whatever can be slotted into a placeholder in a grand theory. But it is real. Elephants, and E. coli and viruses and fungi, and so on all exist, and they are something we attend to because of their importance in, well, life. We have borderline cases like viruses and memes, and how we classify them will depend a lot on what we are doing, but still we must investigate these organisms and objects as they are. Life is a phenomenon.

One of the claims often made is that our theories tell us what the phenomena are. In my book with Malte Ebach, The Nature of Classification, just published, I argue that this is not always true. Some phenomena are handed to us “ready made” as it were, and they set up an explicandum, that which is to be explained and understood by our science. The theoretical explanations are those which we construct to explain these phenomena, and in the process we trim, expand and revise our understanding of what is alive. Once upon a time, the growth of crystals was seen as alive. Now we do not, because we have an account of their growth that is not at all alike the growth of living things with metabolisms. Our theories have revised the phenomena. But many phenomena are resistant to such evaporation or dissipation of the explicanda. Life, I submit, is one of those. It is real, and not to be abandoned because we cannot define it, just as mountains are real although we have no theoretical category for them in plate tectonics. So I think we can say life exists. And if you reject this, I can point to some living things and refute you thus.