The idea that plants have consciousness, sentience, or intelligence would be unthinkable to most people. How could an organism without a nervous system or brain possess these capacities? A minority of scientists are challenging this assumption, however, leading some people to accept the possibility that the spectrum of sentience is more inclusive than previously imagined. At the same time, the proposal of plant consciousness has invited pushback and strong criticism from the scientific community; the notion that plants have a subjectivity – an inner life – and could experience feelings can, understandably, appear absurd.
In spite of curious experiments highlighting the phenomena of plant communication, plant memory, and plant preferences, mainstream plant biologists generally agree that plants do not have consciousness. When the media reports that plants “talk to each other”, this creates the impression that they possess an awareness that makes such vegetal dialogue possible; but such reports can be misleading. The ‘communication’ that plants engage in is not the kind of communication that we are used to experiencing as conscious creatures and that we witness in other sentient animals. Plant communication occurs, as far as we can tell, through purely chemical, mechanistic means – plants can send and receive chemical signals that alter their activity, without needing conscious states to underpin these processes. We see this too in the activity of microbes, lifeforms that most of us would generally assume are non-conscious.
Crucially, plants lack a nervous system, which is the biological architecture considered necessary for sense experience. In a paper published in Trends in Plant Science in 2019, a group of 36 prominent plant biologists stated: “There is no evidence that plants require, and thus have evolved, energy-expensive mental faculties, such as consciousness, feelings, and intentionality, to survive or to reproduce.” They further rejected the notion of plant intelligence because “there is no evidence for structures such as neurons, synapses, or a brain in plants.”
These authors criticise the proponents of ‘plant neurobiology’, a relatively new interdisciplinary field that views plants as information-processing organisms. The term ‘neurobiology’ here is metaphorical; plants don’t have neurons, of course, but proponents use this term as a way to compare electrical signalling in plants with the nervous systems of animals. One of the key researchers in this field, Monica Gagliano, of the University of Sydney, prefers to use the term Plant Cognitive Ecology to clear up this confusing use of ‘neurobiology’ when referring to plants.
It’s also worth stressing that plant neurobiology defined as such is not committed to the view that plants are conscious; this is a view propounded by some plant neurobiologists. Lincoln Taiz, one of the co-authors of the 2019 paper, told The Guardian: “Our criticism of the plant neurobiologists is they have failed to consider the importance of brain organisation, complexity and specialisation for the phenomenon of consciousness.” Taiz adds that plant neurobiology is motivated by environmental concerns. “They [plant neurobiologists] want to raise people’s consciousness about plants as living organisms and reach them on an emotional level. I’m very sympathetic to the motivations, but it is clouding their objectivity.”
It is possible to defend the value and worth of plants without supposing they are loci of consciousness and beings with interests. However, philosophically, some theories do state we can only reasonably have direct moral obligations towards beings with interests since a non-conscious being without interests cannot experience harms or benefits (the animal rights philosopher Tom Regan defends this view). This could still mean that we have indirect moral obligations towards non-sentient life, given that our decimation of plant species and habitats harms non-human animals and humans alike, but I understand why some might be motivated to locate consciousness in plants, as this would revolutionise – as well as deeply complicate – our relationship with them. This reality would force us to radically reconsider the subject of plant ethics, adding weight to the notion that plants have inherent worth independent of their utility to sentient beings.
Plant consciousness would entail respect and care for plants, perhaps even the assignment of specific fundamental rights to them. Nonetheless, for Regan at least, the assignment of basic rights to non-human animals requires more than just awareness. To be a ‘subject-of-a-life’ – as Regan calls it – that is, a being with inherent value that should not be treated as a means to an end, a more complex mental life is needed, which includes things like the ability to experience pain, pleasure, desire, and welfare-interests.
In his paper ‘In Defence of Plant Personhood’ and in his book Plants as Person: A Philosophical Botany (2011), Matthew Hall uses the notion of plant sentience to defend his position that we should think of plants as persons, which will certainly seem outlandish to some. He writes:
It [plant personhood] emerges from an attentive, inclusive approach to plant life that seeks to relate to beings in the world, and in doing so recognizes a range of animate characteristics: including life, communication, agency, autonomy, sentience and volition. Such plant characteristics are increasingly supported by an ever-growing body of empirical observations in the plant sciences. This relational approach privileges and seeks to emphasise connection to beings in the face of morphology and physiology that are radically different from the human.
Hall also underpins the notion of plant personhood with an animistic worldview, the attribution of a ‘spirit’ or sentience to everything; it involves too a particular sensibility towards – and way of relating to – natural entities. The question of whether plants need sentience to have intrinsic value is a complex one and exploring it too deeply may detract from the deeper problem of whether plants can be conscious in the first place. Gagliano, for instance, has criticised the authors of the 2019 paper on plant neurobiology because it “makes strikingly no headway towards a better scientific understanding of what consciousness is.” Indeed, the authors, like many scientists and philosophers, can’t define what ‘consciousness’ is, and if we don’t have that clear definition, how are we meant to decide whether or not plants are conscious? Even if we postulate that a nervous system of a certain complexity is necessary for consciousness, this doesn’t resolve the so-called ‘hard problem of consciousness’, which refers to this mysterious fact of neural states being correlated with consciousness. No neuroscientist or philosopher of mind can tell us why neural states translate into rich subjective experiences, or how the leap is made from inert matter to awareness and feeling.
For this reason, it would be premature and overly confident to conclude plants definitely lack consciousness since it may be the case – as many plant neurobiologists argue – that the information processing seen in plants is consistent with notions of plant cognition and sentience, despite the lack of a brain. Nevertheless, it would be equally rash to conclude plants do possess consciousness based on the evidence available. Alex Jordan, a researcher at the Max Planck Institute of Animal Behaviour, said in a statement to Gizmodo:
I am very happy to see the discussion and debate on non-human consciousness extending beyond mammals to fish, invertebrates, and plants. However, I agree with Taiz [and his co-authors] that the evidence for plants as conscious entities is currently lacking, and while parallels between plant physiology and neurobiology can be drawn, these are not equivalent systems.
There are studies that suggest plants ‘feel pain’ and are conscious, but they are not reputable. Daniel Chamovitz, director of the Manna Center for Plant Biosciences at Tel Aviv University, published a book titled What a Plant Knows. In it, he offers evidence for how plants can see, smell, feel, and have memories. A study published in Oecologica has also shown that plants can ‘hear’ themselves being eaten by caterpillars and respond to these sounds. A common angle in the media was that this was bad news for vegans. However, the study did not indicate that plants could feel pain and suffer, which is what is morally relevant.
In an interview with Scientific American Chamovitz said, “plants had to develop incredibly sensitive and complex sensory mechanisms that would let them survive in ever changing environments.” This is because plants are immobile. They can’t flee from danger or migrate somewhere else in search of more favourable conditions. So they have to be able “to see where their food is…feel the weather,” and “smell danger”. Chamovitz also points out that plants communicate with each other – something which has been bolstered by new studies since the publication of his book – but not by means of anything that could be called an actual language. For example, if a plant is being eaten by aphids, it will release pheromones (scent chemicals) into the air that are picked up by neighbouring plants. It communicates the message of “Aphids! Attack!” And in response, those nearby plants will start making chemicals to help fight off the aphids.
The way that plants respond to their environment is complex and impressive. Yet in spite of recent findings on these sensory capabilities, it is by no means clear that plants can experience pain and pleasure without a brain. Despite endowing plants with more capabilities than mainstream scientists assume they have, Chamovitz states, “Just as a plant can’t suffer subjective pain in the absence of a brain, I also don’t think that it thinks.” In addition, he believes that “purposeful thinking necessitates a highly developed brain and autonoetic, or at least noetic, consciousness.”
Autonoetic consciousness refers to the ability to be aware of one’s existence as an entity in time so that we can mentally place ourselves in the past and future, which lends itself to thinking and acting in a purposeful way. While autonoetic consciousness involves episodic memory (past personal experiences that occurred at a particular time and place), noetic consciousness is identified with semantic memory (a part of long-term memory that processes concepts not drawn from personal experience). Noetic consciousness relates to general knowledge, such as names of colours and other basic facts. Since plants don’t have a highly developed brain, then they may not have the capacity of either autonoetic or noetic consciousness or in turn, an inner life made up of purposes and subsequent goal-directed behaviour.
James McWilliams, writing for The Week, maintains that “recent media attention on the potential emotional lives of plants is – as I see it – little more than pandering to basic scientific illiteracy through semantic sleights-of-hand.” There is a trend of regarding plants as conscious and intelligent. Take the study on plants ‘hearing’ themselves being eaten by caterpillars as an example. Media outlets reporting on this study consistently used the word ‘hear’, but the study itself didn’t use it once. The authors instead stated, “the vibrations caused by insect feeding can elicit chemical defences.” A chemical response to vibrations has been conflated with the experience of hearing. Plant activity can be explained by means of chemical signalling without the need to invoke conscious awareness.
McWilliams believes that there is an ideological motive behind this popularisation of the concept of plant sentience and intelligence, and that’s because “if a reality, [it] undermines the vegetarian ethic.” However, from a utilitarian perspective at least, this wouldn’t be true. After all, if we assume that plants are sentient, and also assume a vegan ethic that is based on reducing as much suffering as possible, then consuming only plants would still result in the least amount of harm since livestock consume more plant crops than humans. If sentience and the ability to suffer exist on a spectrum, then plant suffering would be far more basic compared to that experienced by any animal that we eat. So again, plant sentience doesn’t negate the moral imperative of veganism. Furthermore, if plants are sentient, I can’t imagine any animal rights advocate would take the extreme position that even one’s life must be sacrificed to avoid exploiting sentient life as a resource.
Yale professor Clifford Slayman argues: “‘Plant intelligence’ is a foolish distraction, not a new paradigm.” Of course, we are still ignorant in many ways about plant life. Nonetheless, there is a danger of this ignorance being used to justify a conviction about plant intelligence. To say there is no evidence that plants are sentient may be misconstrued as a dogmatic refusal to accept the possibility that intelligence isn’t restricted to organisms with brains. But it isn’t. It’s about proportioning our beliefs to the evidence available. To argue that plants are sentient because we don’t know that they’re definitely not is a logical fallacy known as ‘appeal to ignorance’. Plants are more complex than previously thought, but the assignment of thinking and purpose to them could be a step too far.
We don’t yet have any persuasive evidence indicating that plants are conscious. Perhaps future research will weaken the mainstream scientific view that there is no feasible way for plants to have conscious experiences, but for the time being, it is difficult to see how plant physiology could support the capacity of awareness. On the other hand, philosophical positions, namely, panpsychism and animism, would challenge this assumption entirely, positing that consciousness does not depend on biological complexity.
To the panpsychist, consciousness is fundamental and universal; it comes in gradations, with the most simple entities (i.e. particles) having the most simple form of consciousness (which various philosophers call protoconsciousness). Plants, then, would have a more complex consciousness but their experiences would still be far removed from a non-human animal’s – and they would be especially nothing like our own. The most appealing promise of panpsychism is that it can solve the hard problem of consciousness outlined earlier: according to this view, simpler forms of consciousness come together to form complex consciousness; and consciousness precedes nervous systems and brains, rather than emerging from them. Animists, meanwhile, posit that plants have their own spirit. Panpsychism, unlike animism, does not have to be framed in any supernatural terms – to be a panpsychist, you don’t have to believe that non-natural entities like spirits exist. Galen Strawson, for example, is one philosopher who defends a physicalist interpretation of panpsychism.
Panpsychism is becoming an increasingly defended position in philosophy of mind while animism is the oldest form of religion and is common to many cultures around the world. Those who hold these beliefs might not consider plant neurobiology so controversial, although the way in which panpsychists and animists claim that plants possess subjectivity and intelligence differs from the claims made by plant neurobiologists. I won’t examine the strengths and weaknesses of panpsychism and animism here, as this discussion was just meant to focus on the scientific disagreements surrounding plant consciousness. From this point of view, it is still hard for the majority of plant biologists to see how plant physiology – and our evolutionary understanding of these organisms – would support the capacity of sentience.