Over at the Opinionator there is a very nice article examining the issue of whether we should eliminate meat-eating in nature if we could. Personally I think that I agree with McMahan’s conclusion that we have more reason to eliminate meat-eating in nature (if we could without great harm) than we do to preserve the various carnivorous animal species. What is striking are the two themes of the comments, or at least the first page of comments…I did not have the strength of will to go through all 11 pages of them. The first is the ludicrous idea that McMahan is somehow advocating the extinction of the Human Race. He very clearly says that humans should stop eating meat and become voluntarily non-carnivorous. The second theme in the comments is that plants have lives and when we eat them we cause them suffering so even being an herbivore is not good enough. Since this is obviously absurd it is concluded that the original claim was also absurd.
Why is this idea so prevalent among people? Our best scientific theories about the world suggest that you at least need some kind of central nervous system in order to have pain or suffering. Carrots have no central nervous system and so cannot suffer. One reason someone might object to this is allegiance to some kind of radical substance dualism. Non-physical minds can be had by anything, even carrots! But just as there is no reason to think that substance dualism is true for humans there is even less reason to think that it is true of carrots. One other line of evidence often cited is that plants react to their environment in ways we didn’t know about 1000 years ago. We have all heard about plants responding to music, “screaming” when in danger, “warning” other trees about fire, etc but isn’t it quite obvious that this is no more evidence that plants feel pain than finding out that a Roomba emits a certain frequency when it is smashed would be evidence for it feeling pain? Plants are alive and react to the environment but unless they have some kind of brain or brain-like system there is no reason to believe that they feel pain or suffer. Reaction to physical damage does not entail that there is pain much less suffering.
Now, I grant that it is conceivable that plants feel pain, and that the issue of whether it is moral to eat them hinges in large part on the answer to this question; if carrots suffered it would be prima facie wrong to eat them. I think I can also grant that we do not know with absolute certainty that plants don’t feel pain, so we cannot rule out that the actual world is in fact a world where carrots feel pain. Even so, if that were the case then McMahan’s argument would have to be put in terms of turning all species into photosynthesizes, or some other non-predatory way of producing energy and the main conclusion would still stand. So it looks like the appeal to plants is doubly off-base. In the first place it is off-base because though plant sentience is a possibility there is no serious reason to think that it is actual and secondly it is off-base because the conclusion would still follow if we amended the argument in a suitable way.