Plants sense and respond to their environment, which is what awareness is

The argument

Proponents of plant consciousness begin from what plants demonstrably do. A plant registers the direction and quality of light and grows towards it. It registers gravity and orients root and shoot accordingly. It registers touch, and some species close or recoil within moments of contact. It registers damage, and responds by altering its chemistry. None of this is passive endurance; it is detection followed by a response fitted to what was detected. The argument holds that this is precisely the structure of awareness. To be aware of something is to take in information about it and to behave differently because of it. If an organism discriminates between states of the world and acts on the discrimination, then the burden falls on the sceptic to explain what further ingredient is missing. On this view, insisting that awareness also requires a brain is not an argument but a stipulation, and one drawn from the single case we know best rather than from any general theory of what awareness must be made of. Proponents press the point by noting how much of this behaviour is flexible rather than fixed. Plants adjust to conditions they have met before, allocate growth towards resources and away from threats, and modify their responses as circumstances change. A purely mechanical system, they argue, would repeat itself; a system that modulates its behaviour in light of what it has encountered is doing something closer to noticing. Because plants continuously sense their surroundings and act on what they sense, this argument concludes, they possess a form of consciousness, even if it is nothing like the human kind.

Premises

Counter-arguments

Critics reply that sensing plus response is far too weak a test, since single cells, immune systems and even thermostats detect states of the world and act on them without anyone attributing awareness to them. On their account consciousness requires an experiencer, and an experiencer requires a nervous system in which signals from across the organism are integrated into a single point of view. Plants have no neurons and no such integrating structure, so there is nothing in them for the detected damage or light to be experienced by.

Rejecting the premises

[Rejecting P1] Detection and response are not in dispute. The premise states the evidence, and the question is what the evidence supports, since nothing in it separates awareness from a mechanism that reacts whenever its trigger is present. [Rejecting P2] The definition offered is satisfied by a thermostat, a camera shutter and a single immune cell, none of which is ordinarily held to be aware. A definition admitting all of those cannot establish the claim it is being used for. [Rejecting P3] That requiring a brain generalises from the human case is a fair challenge, and it does not supply the positive account the argument needs. The objection is not only that plants lack brains but that they lack any structure in which their responses are brought together for anything.