Expert: Dana Krempels, Ph.D. - 9/9/2007

Question
Hello Dr. Krempels, I asked you a question a few weeks ago regarding the hypothesis of pain in plants. But I wonder if you could give me a scientific definition to the concept of pain. Do you think it is "an unpleasant sensory and emotional experience (definition of the IASP)" OR "whatever defensive reaction of an organism to stress"? Furthermore, how can we know if the reaction of an organism to harm is automatic and painless (like blood clotting in animals: we do not choose to heal a wound) OR if the reaction is conscious and painful (like distress of being injuried)? How can we be sure that plants have no consciousness and capacity to feel unpleasantness? Take your time if you are willing to respond...thank you :).

Answer
Dear Jack,

Well, the medical dictionary definition of "pain" runs as follows:

"Pain: An unpleasant sensation that can range from mild, localized discomfort to agony. Pain has both physical and emotional components. The physical part of pain results from nerve stimulation. Pain may be contained to a discrete area, as in an injury, or it can be more diffuse, as in disorders like fibromyalgia. Pain is mediated by specific nerve fibers that carry the pain impulses to the brain where their conscious appreciation may be modified by many factors.

Pain is also a term specifically used to denote a painful uterine contraction occurring in childbirth.

The word "pain" comes from the Latin "poena" meaning a fine, a penalty."

*****
So I think your first definition is more in keeping with that than the second, which really doesn't define pain as much as "defense reaction."  Not really the same thing.

As far as knowing what another organism senses:  we can't.  Just as no person can get inside another human's head to find out exactly what his/her emotions are, what his/her dreams feel like, or how s/he sees different colors, we can only surmise what another organism is "feeling" by observing its reactions to stimuli.

But in doing that, there is always the danger of anthropomorphizing:  assuming that the motivations and reactions of an organism are the same (i.e., have the same evolutionary origins or "feel" the same) as they do in a human.  We cannot make that assumption, either.

It is likely that the more closely related (evolutionarily) two organisms are, the more likely it is that they will share common characteristics, including responses and consciousness about pain.  I would guess that a chimpanzee has a more similar reaction to pain to ours than, say, a centipede.  But that's not to say a centipede doesn't feel pain.  It has a nervous system, a simple brain, and it reacts negatively to harmful stimuli.  

It's not possible to know for sure what other organisms "sense" in response to negative stimuli.  For the moment, given our current technology, we can only assume that a nervous system and simple brain are required for an organism to feel "pain.

Hope that helps.

Dana