Wednesday, June 3, 2009

Plant Sentience

Occasionally, vegans encounter the claim that plants are sentient as a kind of objection to going vegan. The uninformed reasoning suggests that since ‘all life’ is sentient, it doesn’t matter what we eat. Vegans have three replies to this: 1) accept the premise that plants are sentient (no matter how offensive to common sense it is) and argue from there; 2) deny that plants are sentient; or 3) reply with both 1) and 2), as I intend to do here.

First Reply: Plants Are Sentient; Therefore, Go Vegan

Let’s put science and common sense on hold for a couple of minutes and assume for argument’s sake that plants are sentient. Not only that, but let’s take it all the way to absurdity and assume that plants are the most sentient life on Earth.

Even if it’s true that plants are the most sentient life on Earth, veganism would still be the minimum standard of decency. This follows from the simple fact that animals are reverse protein factories, consuming multiple times the protein in plant food that they produce in protein from their flesh and bodily fluids. Cows consume from 9 to 13 times, and pigs 5 to 7 times, the protein they produce, depending on diet and confinement factors. Chickens consume 2 to 4 times the protein they produce, also depending on diet and confinement factors. So the more we’re concerned about the ‘sentience’ of plants, the less we want to contribute to the staggering inefficiencies of cycling plants through animals, and the more reason we have to go vegan to reduce both animal and plant ‘suffering’.

Second Reply: Plants Aren’t Sentient; Therefore, Go Vegan

Let’s now examine the idea that plants are sentient and see why people might believe, contrary to common sense, that plants are sentient, and where they might go wrong.

Equivocation on Sentience

To start with, let’s look at the meaning of the word sentience, because equivocation on the meaning of sentience is often a source of confusion. The definition of sentience in standard usage is an organism’s capacity to experience sensations and emotions. A non-standard definition of sentience, introduced by Robert A. Freitas Jr., and used in the so-called “sentience quotient” (SQ), is the relationship between the estimated information processing rate (measured in bits per second) of each individual processing unit, the weight or size of a single unit, and the total number of processing units. [1]

When a claim is made that plants are ‘sentient’, it is helpful to ask in what sense the claim is being made. Under the SQ definition, plants are ‘sentient’ in that they have an (extremely low) SQ value, but this low SQ value says nothing about sentience under the standard definition. Consciousness sufficient to support experiential sentience almost certainly requires a sufficiently high SQ value in addition to other neuronal properties, neither of which, for example, do computers and plants possess. [2]

Computers have an SQ value that is several orders of magnitude higher than all plants; and animals, including humans, have an SQ value that is up to several orders of magnitude higher than all computers. If computers can’t experience sensations and emotions, then it is almost certainly impossible that plants can, given plants’ extremely low SQ value and a non-neuronal information processing system. As such, it is unreasonable to believe that plants are sentient under the standard (non-SQ) definition.

Plants Are Complex

Another source of confusion regarding plants that leads some people to speculate that they are sentient is that plants are highly evolved and complex organisms that ‘react’ to their environment in surprising ways, especially in larger time scales than we perceive in everyday life. Some plants ‘react’ to insects by releasing deterrent or poisonous chemicals. Some plants release chemicals to deter other plants from growing near them. Some plants are either aggressive or passive in root development depending on whether or not they are around their own species. The Venus Flytrap catches and consumes insects when insects come in contact with tiny hairs that trigger the trap to close.

The confusion arises when the assumption is made that such plant ‘behavior’ is caused by the plants “subjectively experiencing the world through sense data” rather than by insentient hormonal, electrical, mechanical, and chemical processes.

The scientific principle of parsimony strongly suggests that we shouldn’t postulate a complex explanation for phenomena when a simpler explanation will suffice. When autonomic systems in mammals, such as the cardiovascular system, the immune system, and the reproductive system at the level of the ‘behavior’ of sperm in the presence of an egg appear to be reacting ‘subjectively, consciously and intentionally’ to perpetuate either themselves or their host organism, we don’t assume that these systems are sentient independently of their host organism and acting volitionally. We recognize that there are insentient hormonal, electrical, mechanical, and chemical processes that cause various ‘behaviors’ and events to take place. The development of these insentient processes can be explained by tens and hundreds of thousands of years of natural selection, where hundreds of billions of small, genetic mutations and combinations survived or failed to survive based on how adaptive they were. We should apply the principle of parsimony in our assessment of the causes of plant ‘behavior’ similarly.

Sentience and Neurobiology

Neuroscientists have positively confirmed the areas of our neurology (brain stem, limbic system, etc) that serve to provide sentience and complex emotion. All vertebrates and at least some non-vertebrate animals have these nervous system components, providing strong positive, empirical evidence that such beings are sentient, and that most of them have highly subjective, emotional lives. Plants do not have any of these neurological components.

Back to Common Sense

Organisms such as humans, dogs, chickens, pigs, cows, goats, and sheep look, behave, and move in ways that highly suggest sentience defined as the experience of sensation and emotion. Organisms such as plants look, behave, and stay still (unless the wind is blowing) in ways that highly suggest absolutely no sentience (again, defined as the experience of sensation and emotion). Absent an excellent reason to reject such strong appearances we ought to accept them.

If there is any room for debate and legitimate questions on sentience, it is in the biological continuum between insects and bacteria. Insects such as spiders certainly behave and move in a manner that highly suggests at least some degree of experiential sentience. How much sentience comes in degrees, and how sentient certain organisms like spiders are, are difficult questions. But we know beyond any reasonable doubt that vertebrates are sentient; and we know with a very high degree of confidence that plants are not sentient.


As unconscious entities, plants have no subjective, conscious interest that would be morally relevant to whether we kill them for food or other sufficient reasons (e.g. removing/killing them to build a shelter). We should respect plants in the same sense in which we respect the beauty, complexity, and wonder of insentient nature and natural phenomena in general, which entails reducing our impact on them as much as is reasonable, and not destroying them gratuitously. Our moral obligations regarding plants, however, do not compare in kind to our direct moral obligations to vertebrates, whose sentience and conscious, intentional striving for life and survival is obvious to us. Given this eager striving for life and survival of sentient vertebrates, veganism is the minimum standard of decency.



[1] The SQ spectrum ranges from -70 to about +50 and is computed by the formula SQ=log10(I/M), where I is measured in bits/second and M is the mass of the entire processing unit. An SQ of -70 is computed by dividing one bit by the age of the Universe in seconds (10E18 seconds), and dividing that result by the mass of the Universe (10E52 kg). The upper limit of +50 is imposed by the laws of quantum mechanics (see the link to the article below for more information).

Humans have an SQ of +13. The mass of a human neuron is about 10E-10 kg and one neuron can process 1000-3000 bit/s, resulting in +13. Nonhuman animals, from insects to mammals, are said to range from +9 to +13. Computers range from +6 to +9. Plants are said to range from -2 to +1 (the Venus Fly Trap being +1). It is important to note that these are logarithmic values, so that a difference of 5 points is 5 orders of magnitude (i.e. vastly) different.

It should be noted that SQ does not equal sentience under the standard definition of sentience. It’s possible, and even likely, that certain non-human beings could be far more sensitive to certain pain (especially in certain body parts) than humans are, even though they have a lower average SQ. SQ measures only informational processing efficiency, not pain sensitivity, which is dependent on many more factors. We need a sufficiently high SQ to feel pain, which all vertebrates have, but once that high SQ is reached, the other factors affecting pain sensitivity (such as number and sensitivity of nerve endings in certain body areas, etc) have as much or substantially more influence. In some ways, many non-human beings may be far more sensitive to physical and psychological pain than humans are, and that’s one more thing that makes what we do to sentient non-humans so tragic.

The information on SQ came from this article, and if you are interested, there is much more elaboration on SQ in it. Most of the factual details in the above calculations are not source-referenced in the article; however, I did verify the magnitude of the age of the Universe in seconds (quick calculation based on the estimated age of the Universe being about 13.7 billion years) and the mass of the Universe. I also verified the calculations of stated SQ values given the facts presented.

If anyone has good source-references on other facts (or corrections of such facts) presented here regarding SQ (such as the average mass of a human neuron), I’d be glad to update this brief essay with them.

[2] Plants process information via hormones, not neurons. Computers process information via integrated electronic circuitry in semiconductors. Neither hormones nor integrated circuits are known to be capable of producing a subjective experience of sensations. When computer touch screens are activated, for example, the ‘behavior’ of the computer results from programs being objectively and unconsciously carried out via the integrated circuits on the semiconductor devices. The computer is not ‘subjectively aware’ of anything.