I found this really great argument. I want to provide an extended quote so that you can find it in all the other stuff on this forum discussion. I found this at http://forums.earthsave.ca/viewtopic.php?f=15&t=1072&p=22303 posted by user "KelG" on Mon Mar 15, 2010 10:02 pm:
"Humans who believe in a moral code of universal human rights but deny extending rights to nonhumans have two problems. The criteria(s) they use to justify this discrimination (faculty of reason, a soul, divine or evolutionary favor, moral reciprocity, survival of the fittest, individual selfishness, a bundle of characteristics or vaguely defined ones etc.) cannot be proven to be possessed by all humans or lacking in all nonhumans. Secondly, the importance of such criteria can be doubted-- shown not to be objective absolute truth, but subjective arbitrary criteria conveniently determined by those who stand to benefit from the discrimination they wish to justify. Nature (and deities), through environmental phenomenon, weather, earthquakes, etc. cannot be shown to care or favor humans over other lifeforms as an absolute objective fact. This subjectivity means that someone who may discriminate against other humans (which happens despite the laws and philosophy designed to curb such incidents) using criteria that is just as subjective (skin colour, gender, class, religion, survival of the fittest, individual selfishness, etc) cannot be effectively condemned by a human rights advocate who denies rights to nonhumans, since both are discriminating according to subjective criteria of value they deem to be important. The only way for a human rights advocate to consistently argue that one ought to have systemic universal human rights and an ethical code based upon this idea is to extend the concept of fairness and justice to nonhumans as much as possible. Because humans develop ethical codes to govern human behavior, and nonhumans do not appear to employ or require such codes in their social interactions, they benefit from the consistency requirement in human concepts of fairness and justice without needing to reciprocate. To expect them to adhere to human moral contracts in order to be eligible for moral regard is like expecting a blind man to be able to read and then punishing him for not doing so. That moral regard may not be possible or practical in all situations due to particular factors (such as scale or absentmindedness or the inability to be perfect), but since the same is true of human on human interactions, it does not invalidate the merits of the argument or provide a loophole for humans to justify systemic exploitation of nonhuman lifeforms (since one could then justify the same for humans). The common argument for animal rights that values sentience as the criteria of moral worth and attempts to define human discrimination against other species as speciesism, analogous to racism and sexism, does not fully address the philosophical nature of human attitudes towards non human lifeforms, or provide sufficient arguments to counter most if not all attempts at refutation as does the formula presented above. One will find the human supremacy/predation double standard blindspot in any anti-animal rights argument (even the rat/dog/human- who would you save? attack)."