It was once common, in Western societies at least, to think of plants as the passive, inert background to animal life, or as mere animal fodder. Plants could be fascinating in their own right, of course, but they lacked much of what made animals and humans interesting, such as agency, intelligence, cognition, intention, consciousness, decision-making, self-identification, sociality and altruism. However, groundbreaking developments in the plant sciences since the end of the previous century have blown that view out of the water. We are just beginning to glimpse the extraordinary complexity and subtlety of plants’ relations with their environment, with each other and with other living beings. We owe these radical developments in our understanding of plants to one area of study in particular: the study of plant behaviour.
The idea of ‘plant behaviour’ may seem odd, given the association of the word ‘behaviour’ with animals, including humans. When we think of classic animal behaviours – dancing honeybees, dogs wagging their tails, primates grooming each other – we may wonder what there could possibly be in plant life corresponding to this.
One early advocate of the importance of the study of animal behaviour was E S Russell, a biologist and philosopher of biology. In 1934, Russell argued that biology should begin with the study of the whole organism, and conceived the organism as a dynamic unity passing through cycles of maintenance, development and reproduction. These activities are, he said, ‘directed towards an end’ and it is this ‘directive’ activity that distinguishes living things from inanimate objects. Behaviour, according to Russell, was the form of this ‘general directive activity of the organism’ concerned with the relations of the organism to its external environment. This meant that plants quite as much as animals exhibit behaviours. But because plants are sessile (fixed in one place), behaviour is exhibited mainly in growth and differentiation (development of embryonic cells into particular plant parts), rather than in movement, as with animals.
By the end of the 20th century, our understanding of plant behaviour had expanded well beyond growth and differentiation, and it continues to expand. Plant behaviour is, as the botanist Anthony Trewavas puts it, ‘what plants do’. It turns out that they do a lot. Take wounding. Most plants respond to damage to their leaves by releasing volatile organic compounds (VOCs). Some of these VOCs activate abiotic stress-related genes; some have antibacterial and antifungal properties. Some VOCs specifically repel the attacking herbivore with nasty tastes or toxins, and some plants can identify which specific herbivore is attacking, and produce different responses accordingly. Some VOCs attract the predators of the insects that are attacking the plant. Herbivore attack can also induce plants to produce more nectar, encouraging insects away from leaves.
These responses are easily understood as behaviours ‘directed towards an end’ – the end of self-preservation and flourishing. They most likely have an adaptive advantage for the plant. But release of VOCs can also induce neighbouring plants to produce the same response in advance of themselves being attacked, even if they’re a different species. Some experiments seem to show that it is possible to detect plants behaving differently, and favourably, to those that they recognise as their kin (grown from seeds from the same plant). In one experiment, for example, Impatiens plants were grown in shared pots to study how they responded to competition for light above ground and for root space below ground. The researchers found that plants grown in pots with kin plants grew more elongated stems with more branches, whereas those grown with non-kin grew more leaves, blocking other plants’ access to light. The plants thus seemed to cooperate with kin, whereas they tried to outcompete non-kin plants.
Plant being challenges some of the cherished assumptions that have dominated the Western tradition
For some scientists, this research is game-changing, ushering in a novel paradigm in the plant sciences and giving rise to a new view of plant life. Many now believe that the results of this experimental work require us to acknowledge that plants enjoy properties and capacities previously thought to be exclusive to animals or even to humans. Some think that we simply cannot understand what the science is showing us without recourse to these terms.
These extraordinary developments have also caught the attention of philosophers. Philosophy of science is a well-established sub-area within the discipline, and philosophy of biology is a well-established sub-area of philosophy of science. But recently we have seen something emerging that seems to be new: an area of philosophy – ‘plant philosophy’ – dedicated to thinking about plants specifically, not just as a metacritical analysis of the research in the plant sciences but also inspired by it to philosophise anew.
The new plant philosophy has emerged partly in response to this work in the plant sciences, and especially to the new paradigm, because the series of concepts that mark out the new paradigm as new – agency, intention, consciousness, and so on – are already the topic of considerable and long-standing philosophical debate. As soon as attention is focused on plants, broader issues emerge. For it is not just that philosophy is interested in plants; we discover that plant life, or the specificity of plant being, challenges some of the cherished assumptions that have dominated the Western tradition for centuries, if not millennia. Plant philosophy is about more than plants. It is also about how the peculiarities of plant life challenge us to think about our own being in new ways.
To some, the very idea of ‘plant philosophy’ may seem absurd, like some sort of newfangled fad, and certainly you won’t find an entry for it in any recent dictionary of philosophy. But, in fact, there is a relationship between plants and philosophy that is nearly as old as the history of Western philosophy itself. Aristotle provided the basis for theoretical botany when he defined living things as those that have in themselves the capacity for nourishment, growth and decay. Only some natural bodies have the potentiality for life, and the ‘soul’ is the actuality of this potential. Living things, insofar as they are living, thus have a soul (psuchē, which translated into Latin became ‘anima’, whence the distinction between animate and inanimate).
Aristotle distinguished three ‘parts’ of the soul: the ‘nutritive’, the ‘sensitive’ and the ‘rational’. The ‘nutritive’ capacity, the basic principle of life, is common to all living things, including plants. Animals in addition have the ‘sensitive’ part of the soul, while human beings, uniquely, have the ‘rational’ part. Aristotle says that the function of the nutritive part of the soul is making use of nutrition and generation (what would later be called reproduction), both of which are kinds of ‘motion’ or ‘change’. Because even plants have this kind of soul, it has also become known in writings on Aristotle as the ‘vegetative’ or ‘vegetable’ soul (which is why a damaged human being presumed to have no capacity for sensation or thought was said to be in a persistent ‘vegetative’ state).
Aristotle’s idea of the vegetative soul played an important part in botany well into the 17th century. To take just one example, the Florentine philosopher Andrea Cesalpino’s book De Plantis (1583) outlines the first properly scientific classification of plants. He held that the essential nature of plants was in the functions of nutrition and vegetation, given that this was the ‘essence’ of the only kind of soul that they possessed. A system for the classification of plants (which was increasingly needed as global travel revealed the vast number of different plant species) should be based on the essential nature of plants, therefore Cesalpino based his system of classification (at the level of the genus) on the various methods of producing seeds and fruits. In a letter to his patron Alfonso Tornabuoni, he wrote that his success in achieving this system of classification was based on the fact that he combined ‘botanical expertise with philosophical studies, without which no advance can be made.’
Histories of botany see its 19th-century ‘liberation’ from philosophy as a crucial factor in its scientific progress
A commitment to related aspects of Aristotle’s philosophy also drove botanical research. In his theory of the generation of living beings, Aristotle identified the male ‘principle’ with the capacity to impart movement or life by ‘ensouling’ otherwise passive (female) matter. Because his theory of generation was really a theory of the generation of animals, this male power was most often defined as the power to make the sensitive soul. As plants have no sensitive soul, it looks like there could be no male and female in plants. But if, as Aristotle also thought, the male principle imparts life or motion more generally, then, since plants are alive, there must be something like male and female in plants.
This conundrum exercised all the major contributors to the early history of botany. Rather than obstructing research, it drove it forward. While there was nothing obvious in plant physiology corresponding to the sexual organs and substances of animals, researchers thought that there must be something corresponding to them, because plants and animals shared the vegetative soul responsible for generation. Nehemiah Grew, the English botanist credited with discovering the reproductive parts of the flower, did so because he was looking for the Aristotelian vivifying principle of the ‘male office’ (or function) which he connected, albeit confusedly, with pollen.
Today, plant sciences conceive themselves as having nothing to do with philosophy. Indeed, histories of botany see the 19th-century ‘liberation’ of botany from philosophy as a crucial factor in its scientific progress. But this overlooks the extent to which the philosophical commitments of some of the most important botanical researchers shaped their scientific work, and it is naive to think that science proceeds without some basic metaphysical assumptions about the nature of reality.
The insistence on the separation of botany from philosophy also means that we are missing out on understanding the role that botany has played in the history of philosophy as we have traditionally understood it. John Locke (1632-1704) and Jean-Jacques Rousseau (1712-78) were keen botanisers. Was this just a non-philosophical hobby for these philosophers? Or did their study of plants influence their philosophising? In asking these sorts of questions, we return to the new plant philosophy, which seeks both to excavate the history of the entanglements of philosophy and botany, and to renew their collaboration.
One prominent 20th-century figure in this excavation is the English botanist Agnes Arber (1879-1960), who argued for historical and philosophical studies to be part of modern scientific practice. Arber made important contributions to plant morphology in numerous articles and in books dedicated to water plants (1920), monocotyledons (1925) and grasses (1934). In her Natural Philosophy of Plant Form (1950), the philosophical basis of her approach is made explicit. For Arber, plant morphology – the study of plant form – was not just the description of the external features of plants but an investigation into their form in a fuller sense. She looked back to Aristotle’s concept of form (eidos), which she understood to refer to the ‘intrinsic nature of which any given individual was a manifestation.’ Going considerably beyond Aristotle, she took this ‘intrinsic nature’ to encompass the entire life-history of a plant, including changes in the size and outward shape of the plant, thus redefining form as dynamic and refusing to separate it analytically from function. This concept of form was crucial in allowing Arber to see aspects that she believed a purely analytic approach missed.
In her justification of the importance of botanical teratology to the study of ‘normal happenings’, the influence of Spinoza is also clear. Arber describes plant ‘abnormalities’, the ‘revelation of potentialities not usually actualised’, as a demonstration of ‘what a plant can do’, echoing Spinoza’s famous line about ‘what a body can do’. Her theory of the relation between shoot and leaf cites Spinoza explicitly on ‘the urge to self-maintenance’. For Arber, these philosophical sources were the inspiration for botanical hypotheses that she then sought to confirm (often successfully) with the evidence from close observation in the usual scientific way.
Arber’s insistence on the importance of philosophy for botany is shared by some more recent botanists and ecologists contributing to one strand in contemporary plant philosophy. The French botanist Francis Hallé specialises in tropical rainforests and tree architecture (he co-invented the floating raft that first allowed scientists access to the tropical canopy). He is one of a growing number of scientists and philosophers who have begun to wonder what the study of plant life lost in its scientific concentration on chemistry and in ‘reducing the plant to a chromosome number, a sequence of base pairs, a Latin binomial, an electronic image of an organelle, a point on a curve, a bibliographic reference, a datum in a computer memory, a centrifuged residue, or a callus at the bottom of a test tube.’ Alongside this, Hallé advocates in his book In Praise of Plants (1999/2002) for a return to the study of the whole plant ‘from its roots to its flowers, in its soil, with its uses through the ages, that is important, because we need to perceive it with our senses, not only in an intellectual, devitalised way.’
Reflection on the ontology of plant life has implications beyond plants
For Hallé, reflection on what is peculiar to plant life leads inevitably to philosophical questions, which it is equally the job of the botanist as the philosopher to pursue. If the animal individual is a unit (corals are one exception) but the plant develops through reiteration of units, what is the nature of the ‘individuality’ of the plant? Is the idea of the ‘individual’ the same for animals and plants? Might we have to redefine the concept of ‘individuality’ to be able to include plants in its extension?
The French ecologist Jacques Tassin is similarly convinced of the need for philosophy in the pursuit of the understanding of plant life. His book À quoi pensent les plantes?/ What do Plants Think? (2016) sets out from the philosophical question: what is it to be a plant? The question is necessary, according to Tassin, because our ‘irrepressible zoocentrism makes us measure the world according to our animal condition’. To be sure, models based on animal life can be useful in the study of plants, but is there not also a need to find ways to consider the plant according to its own models, closer to its own ‘being’?
It’s clear that reflection on the ontology of plant life has implications beyond plants. In both Western philosophy and everyday life, we tend to think in terms of pairs of oppositions. The dominant sets of such pairs of oppositions, which are presumed to capture in thought something like the structural features of reality, are referred to in a shorthand way by some as ‘Western metaphysics’. They include the pairs inside/outside, matter/form, mind/body, individual/collective, life/death, one/many, male/female. A significant part of 20th-century Western philosophy and related fields such as feminist theory has devoted itself to the critical investigation and sometimes formal ‘deconstruction’ of some of these sets, but they continue to structure much of our thought and action. In many contexts, this is because they are useful and do describe intuitive features of our experience. In winter it is warmer inside my house than it is outside and I want to maintain the inside/outside distinction, thanks very much. These distinctions can survive and tolerate liminal cases (an open window onto a breezy room) in the context of their everyday use. A certain vagueness is built in.
But when the usefulness of these distinctions in some contexts becomes confused with definitive divisions in the fabric of all aspects of reality – when they become ‘metaphysical’ in the strong sense – they can be obstructive and even oppressive (not least because the pairs often function according to an historically established hierarchy, one privileged or valued over the other). If there are aspects of natural and social reality that simply do not bear adequate description within these terms (and yes, natural/social is another one of these distinctions), their presumption will prevent our understanding those phenomena. Another strand of plant philosophy concentrates on the extent to which the being of plants resists adequate description with the traditional terms of Western metaphysics. For example, presuming that plant life can be understood by allocating aspects of it to either side of the individual/collective distinction might not capture what is fascinating about their modular structure or the phenomenon of reiteration. This suggests that the presumed universality of such metaphysical distinctions is false. In his book Plant-Thinking: A Philosophy of Vegetal Life (2013), the philosopher Michael Marder goes so far as to say that plants ‘explode’ Western metaphysics simply by existing: ‘in its very being the plant accomplishes a lived destruction of [Western] metaphysics.’
If it is the case that fundamental features of plant being are not able to be fully understood with the traditional conceptual oppositions of Western philosophy, an unexpected feature of that philosophical tradition emerges. Although we tend to think of philosophy as operating at the highest levels of abstraction and in a purely ‘intellectual’ realm, it seems that its metaphysics might in fact have been cut from the cloth of embodied animal life. Philosophising about specifically human existence needs to begin from the acknowledgement of its finite, embodied psychosocial being. But what would a metaphysics that included vegetal being have to look like?
The redefinition of some terms more usually associated with philosophy than with the sciences – terms like ‘intentionality’, ‘action’ and ‘purpose’ – is already underway in the interpretations of plant behaviour according to the new paradigm. The idea of plant intelligence is central to this. If we begin with the presumption that ‘intelligence’ is an exclusive feature of animal behaviour and that it requires a brain and a central nervous system, or that it is a kind a quantifiable property or capacity of organisms with brains and central nervous systems, then of course we will dismiss the idea of plant intelligence. Advocates of plant intelligence are on strong ground, though, in denying that that presumption has any warrant.
It is perfectly possible to provide definitions of intelligence that apply to plant behaviour, and we already apply the word to nonliving things. If intelligence for biological individuals is defined as ‘adaptively variable behaviour within the lifetime of the individual’, distinguished from genetically determined, developmental processes, then it makes sense to describe plant behaviour as intelligent. It makes sense further to specify the definition for plants. Trewavas accordingly defines plant intelligence as ‘adaptively variable behaviour during the lifetime of the individual’. Examples of this adaptively variable behaviour in plants include directional root growth towards water sources, phototropism (the orientation of a plant towards light) and the release of volatile chemicals as a response to herbivore attack.
The general definition of intelligence used here entails that any organism that was not intelligent in this sense (one that could not adapt its behaviour to its changing environment) simply would not survive. Intelligence according to this definition is thus an intrinsic feature of organisms capable of survival. It is not a definition of a particular feature that one might recognise in some plant behaviours, as if other plant behaviours that lacked it could be classed as non-intelligent. Instead, it treats intelligence as an axiomatic starting point for the investigation of plant behaviour in general. Any controversy here would not be a dispute about whether a certain behaviour was or was not intelligent – the kind of dispute in which each side would attempt to gather empirical evidence to show whether that behaviour met the criteria for intelligence. At issue would be the definition of intelligence itself, which is a philosophical question: what is intelligence? There is an inescapably philosophical dimension underlying the novel paradigm in the plant sciences. Philosophy is baked into this kind of plant science.
Do we really need the concept of agency to understand the phenotypic plasticity of plants?
Of course, we didn’t have to wait for this novel paradigm in the plant sciences (if that is indeed what it is) for the idea of biological intelligence. In that respect, the idea of plant intelligence is not as outré as it first seems. It extends existing thought on biological intelligence to plants. And why not? But the novel paradigm goes much further than this. It is claimed, for example, that plant intelligence requires us also to think of plants as ‘agents’, or as living beings endowed with the power of agency, rather than as a kind of living automaton functioning according to programmed mechanical and chemical responses to external conditions. According to Trewavas and Simon Gilroy writing in 2022: ‘The evidence that plants are agents, that they act purposefully, is indicated by numerous behaviours, most notably plasticity.’
What is it to be an agent? What does it mean to have agency? Do plants have agency in the same way as animals or more particularly humans? Gilroy and Trewavas describe plants as agents that ‘act autonomously to direct their own behaviour to achieve both external and internal goals or norms … while in continuous long-term interaction with the real-world environment.’ For example, in one laboratory experiment, plants grown in dry soil and full light produced larger root systems (maximising access to soil moisture) and narrow, water-conserving leaves with thick cuticles, whereas genetic replicates grown in moist soil and relative shade grew large, broad leaves (maximising photosynthetic surface) with thin cuticles. Plants are said to be agents, then, because ‘they modify their phenotype plastically to improve survival’ and this adjustment is characterised as ‘autonomous action’. But do we really need the concept of agency to understand and appreciate the phenotypic plasticity of plants? What do we gain in applying this concept to plants?
The philosopher of biology Samir Okasha makes a useful distinction between what he calls the ‘organisms-as-agents’ thesis and the ‘organism-as-agent’ heuristic. The first makes an ontological claim about what kinds of things organisms are; Okasha associates this with the opposition to the gene-centric paradigm in biology. The ‘organism-as-agent’ heuristic, on the other hand, is a pragmatic approach that, for the purposes of scientific understanding, treats organisms as if they were agents with goals.
This distinction is not always so clear-cut in practice. The evolutionary ecologist Sonia E Sultan, for example, advocates in her work on plants for what she calls the ‘explanatory strategy’ of ‘a biological agency perspective’, which allows scientists to address some of the explanatory gaps in gene-focused approaches. This sounds like the organism-as-agent heuristic because it stresses the explanatory yield of the presumption of plant agency rather than an ontological concern with what the plant organism is. But Sultan also writes that agency ‘is an empirical property’ of biological systems, ‘a distinctive feature of organisms, the capacity of their constituent systems to respond adaptively to their circumstances’. The agency perspective ‘begins with the observation that organism are agents’ and recognising this helps us to understand how they develop, function and evolve.
But what does the agency perspective really add, and does it capture what the advocates of the new paradigm want to convey with the idea of plant agency and plant intelligence? Does it capture anything specific about plant being?
Sultan suggests that the concept of agency ‘may provide a unifying framework’ for new research in developmental and evolutionary biology on gene expression, development, the nature of inheritance and the basis of adaptation. ‘Agency’ is essentially a term that shifts attention from genes to active response mechanisms, mechanisms leading to changes in development that are in some cases heritable. The ‘agency perspective’ can thus be understood as the name for a research programme that complements gene-focused approaches and not necessarily the attribution of a special capacity to plants. Further, Sultan – like almost all philosophers of biology – explicitly denies that the agency perspective implies in the plant any ‘intention’ to act, much less any conscious intention.
The advocates of the new paradigm for the understanding of plants are not just proposing a new research programme but attempting to build a new picture of plant life with a set of concepts drawn from philosophy and other disciplines. I call this the ‘plant advocacy literature’ because, in addition to its scientific underpinnings, it advocates for and on behalf of plants. Its aim is not just to advance plant science but to make us think differently about plants, to value plant life and accord it more respect.
Much of this literature does not avoid the language of intention and speaks freely of plant ‘choice’. Trewavas for example, claims that research showing that clonal plants on sand dunes grow into resource-rich patches and avoid resource-poor patches makes it ‘difficult to avoid the conclusion of intention and intelligent choice and the ability to select conducive habitats … Intentional choice of habitat is clear.’ In his book Plant Behaviour and Intelligence (2014), Trewavas identifies the meaning of ‘purpose’ or ‘goal’ with ‘intention’ and concludes that plants do indeed intend to resist herbivores and do intend to respond to gravity, but that this merely means that ‘plants are aware of their circumstances, and act to deal with those that diminish their ability to survive and/or reproduce, and thus diminish fitness.’
But it is the anthropocentric language of intention and choice that finds its way into the more popular works on plants, with the Italian botanists Stefano Mancuso and Alessandra Viola even claiming that plants chose a sessile lifestyle and chose to be composed of divisible parts. Plant communication via VOCs is presented to popular audiences as plants talking to each other, and nutrient transfer from older to younger trees via mycorrhizae is described in terms of mothers suckling their young. This anthropomorphising of plants achieves the opposite of the aims of plant philosophy, which are (as far as is possible) to understand plants as plants – in vegetal, not animal or human terms.
What would it mean for us in everyday life to appreciate these beings of another kind living among us?
The challenge for plant philosophy – and it is a huge challenge – is not just to try to achieve some clarity concerning the legitimate use of concepts such as agency and intelligence in the plant sciences. It is also to find ways of conceptualising plant behaviours that avoid both the presuppositions of a gene-determinist approach and the anthropo- or zoocentric approach of some of the plant advocacy literature and in popular media forms. Of course, this reconceptualisation would include insights from existing philosophy of biology, but it would also have to go beyond it.
Philosophy of biology is concerned with questions that, however specific, are general biological questions in the sense that they refer in principle to all living organisms and to processes – like evolution – that are common to them all. Plant philosophy, on the other hand, is concerned specifically with plants, with the specificity of plant life in distinction from most animals and with the implications of this for some general philosophical questions. Do we have to reconceive the concept of the individual to be able to speak of plants composed of reiterated units as individuals? If we began an attempt to construct a general philosophical account of biological agency with plant behaviour, rather than trying to include it afterwards, would this give rise to a novel conception of biological agency? And what would it mean for us in our everyday lives to appreciate these beings of another kind living among us – indeed, keeping us alive?
Contemporary plant philosophy has only just begun to ask these questions. The answers are still wide open.
