That is a very interesting approach. I guess you could draw the line whenever the general public decide to add a specific species to the list. This sounds too complex to tackle. If you give them personhood, could they vote or apply for a driver's license? These species would have to be second class citizens in some fashion. That would open the door to classify some humans as second class citizens.Of course as others pointed out we are vastly more intelligent than most animals. A few species approach some of our mental capacities (without possessing the flexibility, variety, and complexity of our high number of capacities), including dolphins, pigs, great apes, chimpanzees, elephants, crows, dogs, and surprisingly, squirrels and octopuses. As impressive as certain studies are (like the mentioned language and math capacities in dolphins, the abilities that pigs have of playing videogames, and how certain chimpanzees can be trained to perform complex tasks), we haven't exactly found dolphins who composed symphonies like Beethoven's 9th or pigs who have launched spaceships and have touched down on the moon.
However when I think of it, degree of intelligence is not my main focus of concern. I do realize it is the focus of this thread so my contribution is a bit off-topic, but what I focus more, is on sentience, self-awareness, the understand of one's individual destiny and impending death, the ability to discern others as individuals, and similar operations that characterize what I'd see as personhood.
By now, there is no scientific doubt about the fact that some of the other animal species in the planet are also sentient and self-conscious.
I'm not particularly an animal lover, I don't have or desire to have pets, and I'm an omnivore.
However I do believe that certain animals qualify for being considered as persons, and I think they should benefit from some laws granting them some rights such as not being killed by humans for any purpose (be it sport or feeding).
If a dolphin recognizes himself on a mirror, has a name for himself and for his mate, has a concept of family, and mourns when a loved one passes away (and these traits have been more or less convincingly demonstrated in various studies), then a dolphin is a person, and as a person, shouldn't be killed for sport as certain people do especially in the Japanese culture. A person willingly killing a dolphin should be charged with a felony.
I'm not sure how a dolphin would compare to a human in terms of IQ. Certainly their kind of intelligence must be very different from ours and concepts can't be easily transferred from one species to another. Dolphins lack hands to manipulate the environment and can't use fire as a source of energy given their liquid environment. These impediments prevent dolphins from developing a civilization, but certain scientists believe they would be able to develop one, if they had the appropriate means to manipulate their environment.
In any case, even if certain species at best only reached an intelligence equivalent to that of a retarded human child (certain primates have been demonstrated to operate at an IQ estimated at about 50 which would correspond to that of a moderately mentally retarded child), killing a retarded child is a crime, therefore there is no justification for killing a dolphin just because their IQ is perhaps lower than ours (at least, from our side of how to understand and measure intelligence).
So, without going to extremes (for example, I think a chicken and a cow and fish are fairly mindless creatures that are basically live stock raised or hunted/fished for human food consumption, and I have no problem whatsoever with killing and eating them), I believe that at least those animal species found to be sentient, self-aware, and most intelligent, should be protected and left alone to enjoy themselves the way they feel fit, and should be taken out of the human food chain or pleasure hunting targets.
It's a form of kindness that we'd like to enjoy and deserve, if a species of hyper-intelligent space aliens came to Earth one day with an intelligence vastly superior to ours. We wouldn't appreciate it if they killed us just because our IQs were lower than theirs.
I'd want a good body of evidence and scientific consensus to establish a short list of demonstrably sentient/self-aware/highly intelligent species, and I'd pass legislation making it illegal and a felony to kill them.
Where to dry the line? Of course, it's difficult to say for sure, since according to a panel of scientists from Oxford University, most specials of mammals and birds have at least some form of rudimentary sentience.
I'd draw the line fairly high (or else we'd impact on human food resources) - like I said, it would be a short list of species - but even if this left out some arguably sentient species, this would still be better than the current situation in which killing dolphins, pigs, elephants, and great apes in most places is not illegal, at least not outside the realm of endangered species legislation.
My justification for not killing them has nothing to do with endangered species, which I consider to be a rather silly concept (let's leave Mother Nature alone; species have gone extinct by the billions for the entire history of the planet), but rather, with personhood. I don't care if there are 100 million dolphins or 100 of them left. I'd still find wrong to kill any of them, based on the fact that they seem to be able to think and to understand who they are and what is their individual destiny.
I like your thinking very much. It was very fun to read your post. I really liked it.