When most people are faced with the question of why it is alright to harm animals for human benefit (eg for a steak dinner, or for medical science), the most common response I have heard is that it is the intelligence that makes the difference.
I think that the value of a life and the treatment of a life are two different (albeit related) things.
I do not think that it is okay to hurt something just because it is stupid. If that were the case, then we could beat our dogs, and torture mentally handicapped humans. No, the real reason why we must not hurt something is that it can be hurt. If a creature is capable of having experiences with positive or negative valence, then that creature qualifies as a factor in ethical decisions.
This is the difference between sentience and sapience, although the two are often confused. A creature is sapient if it is “capable of higher order thinking” (which is itself a vague definition). Sentience is merely the capacity for a creature to experience reality. And the pain of any one sentient creature is (as far as we know), exactly as painful as the pain of another sentient creature, regardless of intelligence (or “sapience”).
The principle is simple: If something can be tortured then it shouldn’t be.
What if the death is painless and instant?
Well then, we can’t really say that we are causing it any negative experiences. In which case the real harm caused by the death will be social: the friends and family left behind will mourn them.
This idea has the very interesting implication that it is actually very hard to unenthically kill a non-social animal, provided you do it painlessly. If the species is very solitary and not at all empathic in nature, then its death will cause no grief to other sentient creatures.
Of course, the majority of the creatures that humans regularly kill are very social mammals such as pigs, cows, goats, etc. So the social impact of death is actually very commonly an important factor in the ethical choice.
But there are also other indirect consequences for killing social creatures. For example, if you kill the leader of a pack, turmoil may ensue. If you kill the only hunter, the rest may starve. And if you kill a scientist, then you may have delayed the discovery of the cure for cancer.
So while intelligence is not a factor in whether or not you harm the individual, it is a factor in whether or not you indirectly harm the group. For example, going around killing all of the intelligent, altruistic people in the world is clearly far worse than going around killing all of the selfish imbeciles you can find. And the reason that it is different is not because intelligent humans feel more pain as you kill them. It is because of the indirect consequences: the benefits that you postpone or cancel for those left behind.
And similarly, we might say that when we kill a cow or a rat, we aren’t postponing any scientific discoveries or technological advances or other improvements to other sentient individuals. On the other hand when we kill a human, we are indeed postponing such things…
Or are we? Is the average human really that likely to improve things? Do we give our species too much credit? What do you think?
Wikipedia. (2013). “Sapience”. Online @ http://en.wikipedia.org/wiki/Sapience#Sapience
Wikipedia. (2013). “Sentience”. Online @ http://en.wikipedia.org/wiki/Sentience