Could plants be conscious? What are the limits of consciousness and pain?
Scientific American recently reviewed a book titled "Planta Sapiens". The title gives it all away, and the review was quite positive, with statements like:
"Our senses can not grasp the rich communicative world of plants. We therefore lack language to describe the 'intelligence' of a root tip in conversation with the microbial life of the soil or the 'cognition' that emerges when chemical whispers ripple through a lacework of leaf cells."
This is provocative indeed! What if plants really do have a secret life and suffer pain with our every bite and swing of the scythe? What of our vaunted morals and ethics then?
I am afraid that I take a skeptical view of this kind of thing, so let's go through some of the aspects of consciousness, and ask how widespread it really is. One traditional view, from the ur-scientific types like Descartes, is that only humans have consciousness, and all other creatures, have at best a mechanism, unfeeling and mechanical, that may look like consciousness, but isn't. This, continued in a sense by B. F. Skinner in the 20th century, is a statement from ignorance. We can not fully communicate with animals, so we can not really participate in what looks like their consciousness, so let's just ignore it. This position has the added dividend of supporting our unethical treatment of animals, which was an enormous convenience, and remains the core position of capitalism generally, regarding farm animals (though its view of humans is hardly more generous).
Well, this view is totally untenable, from our experience of animals, our ability to indeed communicate with them to various degrees, to see them dreaming, not to mention from an evolutionary standpoint. Our consciousness did not arise from nothing, after all. So I think we can agree that mammals can all be included in the community of conscious fellow-beings on the planet. It is clear that the range of conscious pre-occupations can vary tremendously, but whenever we have looked at the workings of memory, attention, vision, and other components assumed to be part of or contributors to conscious awareness, they all exist in mammals, at least.
But what about other animals like insects, jellyfish, or bacteria? Here we will need a deeper look at the principles in play. As far as we understand it, consciousness is an activity that binds various senses and models of the world into an experience. It should be distinguished from responsiveness to stimuli. A thermostat is responsive. A bacterium is responsive. That does not constitute consciousness. Bacteria are highly responsive to chemical gradients in their environment, to food sources, to the pheromones of fellow bacteria. They appear to have some amount of sensibility and will. But we can not say that they have experience in the sense of a conscious experience, even if they integrate a lot of stimuli into a holistic and sensitive approach to their environment.
The same is true of our own cells, naturally. They also are highly responsive on an individual basis, working hard to figure out what the bloodstream is bringing them in terms of food, immune signals, pathogens, etc. Could each of our cells be conscious? I would doubt it, because their responsiveness is mechanistic, rather than being an independent as well as integrated model of their world. Simlarly, if we are under anaesthesia and a surgeon cuts off a leg, is that leg conscious? It has countless nerve cells, and sensory apparatus, but it does not represent anything about its world. It rather is built to send all these signals to a modeling system elsewhere, i.e. our brain, which is where consciousness happens, and where (conscious) pain happens as well.
So I think the bottom line is that consciousness is rather widely shared as a property of brains, thus of organisms with brains, which were devised over evolutionary time to provide the kind of integrated experience that a neural net can not supply. Jellyfish, for instance, have neural nets that feel pain, respond to food and mates, and swim exquisitely. They are highly responsive, but, I would argue, not conscious. On the other hand, insects have brains and would count as conscious, even though their level of consciousness might be very primitive. Honey bees map out their world, navigate about, select the delicacies they want from plants, and go home to a highly organized hive. They also remember experiences and learn from them.
This all makes it highly unlikely that consciousness is present in quantum phenomena, in rocks, in bacteria, or in plants. They just do not have the machinery it takes to feel something as an integrated and meaningful experience. Where exactly the line is between highly responsive and conscious is probably not sharply defined. There are brains that are exceedingly small, and neural nets that are very rich. But it is also clear that it doesn't take consciousness to experience pain or try to avoid it, (which plants, bacteria, and jellyfish all do). Where is the limit of ethical care, if our criterion shifts from consciousness to pain? Wasn't our amputated leg in pain after the operation above, and didn't we callously ignore its feelings?
I would suggest that the limit remains that of consciousness, not that of responsiveness to pain. Pain is not problematic because of a reflex reaction. The doctor can tap our knee as often as he wants, perhaps causing pain to our tendon, but not to our consciousness. Pain is problematic because of suffering, which is a conscious construct built around memory, expectations, and models of how things "should" be. While one can easily see that a plant might have certain positive (light, air, water) and negative (herbivores, fungi) stimuli that shape its intrinsic responses to the environment, these are all reflexive, not reflective, and so do not appear (to an admittedly biased observer) to constitute suffering that rises to ethical consideration.
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