Carol Cleland has been among the most influential voices in the literature on the nature of life for decades, in both philosophical and scientific circles. This book is, in part, a culmination of her earlier groundbreaking ideas, collecting them in one place and highlighting their interplay. It also breathes fresh life (so to speak) into this literature, bringing a deep understanding of the science of astrobiology to bear on the philosophical issues.
This book is an ambitious response to over 2,000 years of debate about the question “What is life?” There are, by some counts, over 100 proposed answers on offer. There is no consensus within disciplines with a stake in distinguishing the living from the nonliving, let alone across disciplines. A more recent meta-debate regards whether and how we should even try to answer that question. Cleland speaks to both the debate and the meta-debate.
Methodologically, the book approaches the nature of life by integrating relevant scientific material for philosophers with philosophical material for scientists. This is no easy task, given the topic’s deep reach into technical issues on both sides (think: early Earth biochemistry for philosophers; reference and natural kind terms for scientists). Cleland does a commendable job, including signaling points where the material might be rough going for an uninitiated audience. Although not a primary aim of the book, I appreciated its condensed tours through core topics such as natural kinds, the syntactic and semantic views, incommensurability, and theory change. This would be a good book for introducing students to some classic philosophical debates through the lens of the nature of life.
The book is organized around several themes, starting with the link to antiquity. A central argument regards the “enduring legacy of Aristotle.” Cleland traces a historical thread of ideas leading from Aristotle’s self-nutrition and other key functions of life through to the contemporary focus on self-organization and reproduction as hallmarks of life. A second theme regards how we approach the question “What is life?” in the first place. Cleland argues that we should be after a theory of life, not a definition, because definitions are counterproductive to our interests in that question; they are “far more likely to blind us to unfamiliar forms of life than to facilitate their discovery” (75).
Definitions of life face well-known counterexamples: if metabolism is sufficient for life, then apparently candle flames can be living; if reproduction is necessary, then sterile hybrids like mules are disqualified. Cleland argues—through ongoing comparisons between the questions “What is life?” and “What is water?”—that the definitional approach to life is flawed in a deeper way. Both questions, she says, are best answered with a theoretical identity statement. Theoretical identity statements tell us something important in response to a “What is X?” question, but unlike definitions, they do not supply necessary and sufficient conditions for membership in the kind X. “Water is H2O” is a theoretical identity statement grounded in the well-supported chemical theoretical frameworks which shape our contemporary understanding of water and which are, like all scientific theories, open to revision.
So where do we find our theoretical identity statement(s) for life? This calls for a sufficiently well-developed theory. This is where another central theme of the book, and one of Cleland’s important contributions from earlier work, comes in: the “sample size of one” problem. So far, we have only a single sample of life on which to base our theorizing. Reflecting on the incredible diversity of life as we know it, this might seem like a puzzling thing to say. The point is that all of those endless forms of Earth life—that we know of, anyway—share a common ancestor and core biomolecular features. We have good reasons to think that the origin of life on Earth around 4 billion years ago involved many contingencies. Given our single sample, we are on shaky grounds to say which features of life as we know it are accidental and which are universal. Cleland thinks our current sample is likely unrepresentative of life generally, in a variety of ways, which she reviews in light of some beautifully detailed yet accessible discussions of origin of life research. She concludes that for now, unlike our answer to “What is water?,” our answer to “What is life?” is a moving target that will change as we seek to increase our sample size (more to follow on how we might do so).
A prerequisite for well-developed theories, Cleland argues, is starting with fruitful ontologies that hit the “Goldilocks zone” of abstraction, supporting generalizations while reducing exceptions. She thinks our theorizing about life has a long way to go on this front, partly because of our entrenched focus on self-organization and reproduction. We’re left with two questions: How do we know when it’s time to relax our hold on a given ontology and entertain new ones? And to which properties should we shift our focus in theorizing about life if we relax our hold on self-organization and reproduction?
Cleland discusses but does not fully answer these questions; both are worth pursuing further. In response to the first question, she recommends a methodological stance called the monist stance, partly as a rejoinder to the recent trend toward pluralism about life. Her monist stance is a heuristic for approaching theory development and is ultimately compatible with pluralism; the key recommendation is to seek theories, and ontologies to ground them in, that maximize scope and minimize exceptions.
In response to the second question, Cleland suggests microbes as a compelling starting point for grounds to overhaul our views of life’s core properties. She worries that efforts to understand life have focused too heavily on an unrepresentative subsample of our already limited single sample: animals and other macroscopic life-forms.
Plenty of astrobiologists focus their attention on early or very ancient life on Earth as grounds for thinking about life in the universe. Given that early and very ancient life was all microbial, such scientists, at least, cannot really be fairly charged with overlooking microbes. But although philosophers of biology have been told before to pay more attention to microbes (O’Malley and Dupré Reference Cleland2007), this is an apt point to reinforce in the context of theorizing about the nature of life itself. It would be interesting to dig deeper into understanding to what extent all theorizing about life’s origins and distribution in the universe, even when microbes are put in the spotlight, is ultimately swayed by our thinking about macroscopic life; I don’t know the answer. In any case, anyone still guilty of ignoring microbes will benefit from reading chapters 5 and 6, which contain engaging discussions of microbiology, microbial classification and remarkable surprises about life from the microbial world.
Cleland makes a case for shifting focus to the microbial world in our “hunt for new ontologies” for theorizing about the nature of life. Compared to focusing exclusively on animals and plants, this certainly reveals a far richer understanding of the possibility space for what life forms can look like, what they can do, and how they make a living. I wonder whether and how this shift in ontological hunting grounds would lead us to focus ultimately on core properties of life other than self-organization and reproduction. Those are both activities that the vast majority of microbes and macrobes do their own versions of—with a few exceptions and intriguing borderline cases, as with just about any other “universal” feature in biology—and in a remarkable number of different ways. Which alternative fundamental properties of life could our microbial reevaluation lead us to? This would be a fascinating question to take up further.
The final chapters discuss prospects for increasing our sample size beyond one: artificial life (ALife), finding life beyond Earth, and finding a microbial “shadow biosphere” on Earth.
ALife is a multidisciplinary research area with many overlapping aims. Cleland concerns herself with “strong ALife,” which aims to create living artificial systems in software, hardware, or the wet lab. She is skeptical of its promise as a source of genuine further samples of life because ALife creations are closely based on our thinking about Earth life and tend to fall outside the Goldilocks zone of abstraction: they are either highly abstract and functional (in the software and hardware cases) or grounded in the biochemistry of life as we know it (in the wet lab case).
Cleland seems to at times affirm, and at other times back away from, the view that life is ultimately a functional phenomenon. In her discussion of why hardware-based ALife is too abstract to capture life’s core properties, she stresses reasons to think that life is ultimately more physicochemical than functional. Regarding ALife in the wet lab, she suggests that if researchers were to someday create a synthetic cell sharing functional characteristics with life as we know it but with a sufficiently different molecular basis, “then there is little doubt that it would qualify as a truly alien form of life” (170). Both points are ultimately consistent with her overall message that our theory of life is a moving target: we should keep an open mind, for now, about how functionally (or not) to understand life. This discussion points to some important and subtle open questions about where and how to find the Goldilocks zone when we are pulled in both physicochemical and functional directions, depending on where we look.
Could astrobiology’s search for life beyond Earth solve the problem of a sample size of one? Cleland thinks the prospects are better than with ALife, and here she drives home a point she has made in influential earlier work: astrobiologists should employ well-reasoned but tentative criteria for life, rather than strict definitions, in their search for biological anomalies and life in the universe.
The book ends with a captivating discussion building on yet another of Cleland’s previous contributions. She coined the now widely used term shadow biosphere in earlier work to refer to the prospect of microscopic life on Earth that lacks a common ancestor with life as we know it and that is sufficiently different that even our fanciest microbe-detection techniques have not found it yet. Such a discovery would be awesome and revolutionary. It’s worth noting here, though, that were we to detect such microbes, it would remain to be seen whether this was truly a second sample of life in Cleland’s sense or just motivation to overhaul our big picture of the phylogeny of life as we know it in unprecedented ways, akin to (but more deeply than) what happened when we learned about Archaea. In other words, apparent shadow microbes could turn out to be either a second sample of life or evidence of an earlier last universal common ancestor than current views posit.
Cleland discusses a range of puzzling and fascinating anomalies that scientists have been unable to classify as biological or abiological in origin, including geological phenomena tantalizingly suggestive of microbial activity. Microbes could, in principle, be behind these findings, but so far there is no conclusive evidence of them. Although Cleland does not overstate the prospects of finding a shadow biosphere, she ends the book on the optimistic note that we should not dismiss the possibility that “life as we don’t know it” could be behind these anomalies, and we should keep searching for more. I ended the book with a sense of amazement at the range of anomalous life-suggesting phenomena on Earth and beyond and at the fact that more time and money haven’t been poured into figuring them out.
This book draws readers into the excitement of thinking about the nature of life not in the abstract, but through engaging with the theory and practice of astrobiology. There is something to be gained here for a wide range of readers, philosophers and scientists alike, familiar or unfamiliar with Cleland’s earlier pioneering contributions to the literature on life.
