Plant Minds

Do plants have minds? Until he stumbled upon recent scientific work in plant intelligence, Maher—like the majority of philosophers of mind—didn’t think so. In Plant Minds: A Philosophical Defense, Maher explains how he came to change his opinion through reconsidering both what plants can do and what minds are (pp. 2f.). It is primarily prejudices about the latter—most importantly, ‘the idea that having a mind requires the capacity to represent the world’ (p. 3)—that prevent us from acknowledging plant minds. If we abolish representationalism, adopting an enactivist theory of the mind instead, we can happily welcome plants into the class of mindful beings. Or so Maher argues.

Maher’s book is organized into six chapters. The first chapter, ‘Do Plants Have Minds?’, motivates the book’s topic through a reflection on concepts of the mind from antiquity to the present. Contrary to the impression created by contemporary philosophy of mind, there are a variety of interpretations of what a mind is, some perfectly compatible with plants having minds. In fact, only Cartesian dualism and identity theory seem to rule out this possibility, leaving Aristotelianism, mechanism, Darwinism, behaviourism, and computationalism as potential allies. Pursuing the question of whether plants have minds is thus perfectly reasonable and worthwhile.

In the four following chapters, Maher investigates how plants perform with respect to those ‘interconnected abilities’ (p. 22) that—apart from the ability to think—have, historically, been associated with the concept of mind: ‘perception, feeling, memory, and agency’ (p. 22). This requires engaging more closely with the latest research on plant physiology.

The second chapter, ‘Perceiving’, looks at tropic plant movement whereby plants respond to directional environmental stimuli, for example, phototropism and gravitropism. However, Maher argues that these do not qualify as instances of perception insofar as perception is commonly assumed to require representation. Only if the causal theory of representation were true could plants be said to represent aspects of their environment that have a causal effect on them. But the causal theory of representation cannot be true because not every effect is a representation and, equally, not every representation is veridical.

Nastic movement whereby plants respond to non-directional environmental stimuli, such as thigmomorphism, prompts the question of whether plants have qualia, which is addressed in the third chapter, ‘Feeling’. The answer, we are told, depends on what we think qualia require. If some version of identity theory is true—that is, if ‘neural happenings are necessary and sufficient for qualia’ (p. 62)—then plants are disqualified as they don’t have neurons. If a representational theory of qualia is true, then plants are again disqualified because they don’t seem to have representations. There are, however, reasons to reject both of these theories. First, proponents of the new discipline of plant neurobiology, which is founded upon the thesis that plant physiology is ‘significantly analogous to neurophysiology’ (p. 61), claim that neurons may not be necessary for qualia. Second, at least some qualia may be non-representational. The latter resonates with an enactivist understanding of the mind, according to which ‘qualia (and consciousness more generally) are rooted in the ways things matter to an organism, which is rooted in its being an autopoietic and adaptive system’ (p. 73).

Phenomena such as phototorsion—twisting towards the sun—or photoperiodism—growing in response to light intensity—suggest that plants remember facts about their environment, that is, that they store, encode and retrieve information. The fourth chapter, ‘Remembering’, discusses whether this could be the case in the interesting sense of strong, semantic information. This would mean attributing representational states to plants. However, Maher argues, the functional theory of representation that appears to support the case for plant representations is unacceptable: having the function of being caused by something is not sufficient for representing that thing (p. 93).

The fifth chapter, ‘Acting’, proceeds from the observation that ‘plants appear to initiate things on their own, and for their own good’ (p. 100). For example, chemomorphism is the phenomenon whereby plants release chemicals as a pre-emptive defence against attackers. The common assumption that voluntary action, as opposed to mere reflexes, requires the activity of a mind is interpreted by the computational theory of the mind as involving computations, in the sense of ‘rule-governed manipulations of representations’ (p. 105). However, as discussed, it is highly doubtful that plants possess genuine representations. Hence, if the computational theory of the mind is true, plants do not have minds.

Having presented ‘the best case for thinking that plants do not have minds’ (p. 3), the sixth and final chapter, ‘Mind in Life’, undertakes to present a positive argument in favour of plant minds. This is where enactivism re-enters the scene. Enactivism rejects representationalism, claiming instead that ‘autopoiesis-and-adaptivity suffice for having a mind’ (p. 120) and that to have a mind means ‘to disclose (bring forth or ‘enact’) a world of things that have significance (meaning or value)’ (p. 114). Insofar as all living systems are autopoietic-and-adaptive, this amounts to what Maher (following Thompson [2007]) calls the ‘mind in life’ thesis: ‘that all living things have a mind’ (p. 121). After defending this thesis against a number of objections, Maher concludes that enactivism ‘makes it plausible that plants and other organisms have minds’ by calling into question the mainstream conviction ‘that mind has nothing to do with life’ (p. 127).

Maher’s book nicely demonstrates the relevance of plant intelligence to contemporary philosophy of mind. Recent empirical research revealing the sophisticated organization and behaviour of plants, including their astonishing plasticity (see, for example, Sultan [2015]), poses a challenge to over-intellectualist views of intelligence, cognition, and whatever else it is that the concept of a ‘mind’ is meant to designate—a challenge that must be taken seriously. The appeal to the historical flexibility of the concept of a mind is constructive in this respect: it may turn out that we have to revise our views about the metaphysical nature of mind in light of scientific discoveries once more, in accordance with a philosophical agenda currently gaining traction under the title ‘metaphysics of science’.

Yet, I have to admit that I remain unconvinced by both Maher’s negative case against and his positive case for plant minds. This has to do with the book’s overall perspective and structure, which make it evident that it was written by someone trained in classical philosophy of mind and is intrigued by plants, rather than by a philosopher of biology, cognitive scientist, or even ‘plant neurobiologist’. As a result, some of the central motifs driving, and challenges faced by, actual biological and philosophical research in plant intelligence end up being obscured.

In accordance with his core thesis that the question of whether plants have minds hinges upon the question of whether having a mind requires having representations, Maher contrasts the logical structures of the negative case with the positive case for plant minds as follows:

1 Having a mind requires having representations.
2 But plants don’t have representations.
3 Therefore, plants don’t have minds. (p. 106)

1 Autopoiesis-and-adaptivity suffice for having a mind.
2 Plants are autopoietic-and-adaptive.
3 So, plants have minds. (p. 120)

Generally, if the book’s aim is to convince the reader of the positive case, one might wonder why it deserves only one chapter when a total of four chapters are dedicated to the negative case. Supposedly, this reflects the dynamics of the process by which Maher came to change his mind about plant minds, which began with the conviction that there is no such thing as plant minds. Does Maher present the negative case in such strong terms to justify his initial adherence to a theory that he now regards as mistaken? Or does he simply want to do justice to orthodox representationalist views in contemporary philosophy of mind? Is the idea that these views, despite having a lot going for them, must be rejected? But if the latter, a case wasn’t made for why representationalism might have been considered attractive in the first place.

To make the negative case against plant minds a strong one, it seems what is needed is a defence of the first premise, that there is no coherent and meaningful way to think of a mind other than in terms of representations. I doubt that Maher has presented such a defence (and not surprisingly so, as he opposes representationalism). What the reader is given are pieces of history—a story about the rise of computationalist and representationalist approaches in the philosophy of mind—together with a detailed defence of the argument’s second premise, that plants do not happen to have those representations deemed necessary by the received view of the mind. However, if the reader is already unconvinced by the representationalist approach to the mind, the apparent lack of representations in plants will not lead to a denial of plant minds. From the perspective of cognitive science, where embodied approaches have become increasingly influential in recent decades, Maher’s appeal to a putative representationalist consensus appears outdated and a poor substitute for arguments in favour of a representationalist approach.

At the same time, we may wonder how convincing Maher’s denial of representations in plants really is. The newly created disciplines of plant neurobiology and the philosophy of plant neurobiology (see Calvo [2016]) frequently attribute computations (Trewawas [2003]) and representations (Gagliano [2017]) to plants. Presumably, Maher thinks that what these scholars pick out with these terms are not ‘genuine representations’ but ‘merely weak information’ (p. 112). But if so, it would have been helpful to learn how and why, in his opinion, some friends of plant intelligence go wrong in attributing (too) strong representations to plants or why the weak notions of representation they employ are not interesting. Obviously, talk of computation and representations in plants—alongside plant ‘cognition’, ‘intelligence’, ‘learning’, ‘memory’, ‘perception’, and ‘feeling’—has been robust and interesting enough to prompt the invention of a novel field of research under the (admittedly controversial) name ‘plant neurobiology’. Indeed, the most accurate diagnosis of the state-of-the-art is that it is an open question whether this discipline should adopt a representationalist or a non-representationalist line of thought (Calvo [2016]). It is a pity that Maher doesn’t discuss this question in his ‘philosophical defense’ of plant minds (as the book’s subtitle reads), despite its focus on the notion of representation (plant neurobiology is mentioned only briefly in the third chapter, as indicated above).

Such a discussion would also have been helpful to Maher’s positive case for plant minds. To convince, it required a compelling defence of its key premise—that autopoiesis and adaptivity are sufficient for having a mind—rather than a defence of the second premise—a four-page reflection on ‘how thoroughly autopoietic (in Thompson’s broad sense) plants are’ (p. 115). That plants are autopoietic systems is hardly controversial, but that this suffices for them to have minds is, even among supporters of plant intelligence. For example, Trewawas’s ([2002]) seminal Nature article on plant intelligence is entitled ‘Mindless Mastery’, expressing his assumption that there can be computation and representation, and in this sense cognition, but that because plants don’t have brains, they don’t have minds (Trewawas [2003], p. 17). On the other hand, even if we reject the identity theory and accept some version of a plant–animal homology thesis (Calvo [2016]), there is still room to disagree about the mindfulness of plants. Maher himself admits that we commonly ‘use “mind” for a thing’s ability to think and to remember’, which is why we ‘should not hastily equate’ Aristotle’s ‘psuche’, and our concept of a ‘mind’ (p. 4). Indeed, our modern concept of mind comes closer to what Aristotle calls ‘nous’, that is, intellect or reason. Why should we switch to a wider notion of mind that includes both Aristotle’s nous and psuche, the latter being understood, Maher suggests, as ‘orderliness toward a goal’ (p. 5)?

This is not to say that there are no good reasons to do so. Indeed, I think there are, and Maher indicates these when complaining that ‘contemporary theorizing about the mind is “lifeless”’ (p. 121), and insisting that ‘our conscious minds—what we sometimes lazily slide into thinking of when we think of intelligence—are far from the only intelligent thing around’ (p. 126). Surely, a wider, integrative notion of mind more readily reflects the continuity between human organisms and non-human organisms that ‘are more sophisticated, more intelligent, closer to us than we commonly think’ (p. 126)? But this concern is not novel. What is novel, and remains in need of work, is the claim that plants are to be included in the class of mindful non-human organisms.

From the third chapter, it becomes apparent that Maher himself is primarily interested in attributing qualia to plants. Curiously, his corresponding Thompson-inspired appeal to enactivism—‘Enactivism implies that for any organism, there is at least a proto-feel to its encounters with things’ (p. 73)—directly contradicts Thompson’s ([2007], p. 162) thesis that an organism’s ‘immanent purposiveness does not entail consciousness’, not even in the sense of ‘a prereflective self-awareness’. Maher indicates, in a footnote, this tension between his and Thompson’s position without resolving it. Note that Thompson nowhere in his book offers an in-depth analysis of plants. Neither does Heidegger, to whose concept of ‘disclosure of an umwelt’ (p. 127) Maher refers in an attempt to motivate his rejection of a representationalist concept of the mind (recall Heidegger’s ([1995]) tripartite hierarchy of ‘worldless’ things, ‘world-poor’ animals, and ‘world-building’ humans). The lack of any explicit discussion showing how Thompson’s and Heidegger’s arguments also apply to plants, or could be extended to plants, leaves unconvincing Maher’s thesis that plants’ and humans’ encounters through feeling with a meaningful world, despite one being ‘not as robust, complex, or sophisticated’ as the other, ‘are nevertheless helpfully placed together along a spectrum’ (p. 73; but see Calvo [2017]) for an empirically grounded defence of a similar thesis).

Maher does a good job of demonstrating how our views on plant minds depend on conceptual presumptions about minds in general. However, a closer engagement with both enactivism and existing philosophical reflections on plant intelligence (‘philosophy of plant neurobiology’) would help to avoid the impression that answering the question of whether plants have minds is primarily a conceptual matter. A truly convincing case for plant minds must do more than re-define the concept of ‘mind’ so as to accommodate plants. And, on pain of circularity, it must do more than reject representationalism about minds on the assumption that it excludes plants from having minds.

Let me close by admitting that what may worry experts dealing with plant intelligence may be of little importance to the non-experts for whom Maher’s book, part of the ‘Routledge Focus on Philosophy’ series, is written in the first place. The stated aim of this series is to tackle ‘big topics in a digestible format’ so as to ‘open […] up important philosophical research for a wider audience’ and the blurb for this book states that ‘Plant Minds aims to help non-experts begin to think clearly about whether plants have minds’. If you don’t expect more than that, then Maher’s book is a perfectly worthwhile read that is only sweetened by Jim Sias’s endearing illustrations.