In a previous discussion, there was some debate over the difference in value between animal life and human life. I thought it might be worthwhile to lay my cards on the table over the distinction I place on the two.
I could make a theological argument here, but I don't think that would be very helpful. Not only because most of my readers aren't Christians, but also because it would make for a very short post. A quick reference to the first few chapters of Genesis, and there you go. Humans > animals.
For the philosophical, non-religious approach, I can sum up my argument in two words: Intellectual Capacity.
Though humanity displays varying degrees of it, intelligence is the factor that distinguishes us from the lower life forms. A few different manifestations of this capability of humanity are worth expanding on:
Science, Art/Music, Culture - Some of the greatest accomplishments of mankind are summed up in those concepts. The quest to explore the natural order and better the lot of humanity through those discoveries is only possible because of mankind's capacity for rational thought. The fine arts are sometimes rationalized as being evolutionarily similar to mating rituals in other species, be it colorful displays or sound-based mating calls. I think, however, that the reliance of abstract principles in art and music take it well beyond simply an attempt to impress the opposite sex and increase mating potential.
Conservation - Mankind is unique amongst the denizens of this planet in its approach to conservation. Yes, people do argue over the best way to implement conservatory principles. Some want to consume away and let scientific advancement deal with the consequences. Others would prefer that we level our cities and do nothing to disrupt the natural balance anywhere. But inbetween those two extremes, mankind agrees that nature should be preserved in some way. We recognize that our dependence on the world around us requires us to ensure its survival so that we, as a species, will also survive.
Contrast this with animals. Nature has many controls built in so that most ecosystems are in balance. Food supply, predatory population, reproductive capacity; these things keep animal populations in check. But as we see with deer, as an example, if you remove one of those controls, the animals have no sense of conservation. They will eat and breed until their territory can no longer sustain them. They then either die or spread out. Mankind, at least in part, recognizes that it cannot live in that manner.
Morality and Ethics - I'd consider this one of the most important aspects of intellectual thought that sets man apart from animals. Animals are driven, by and large, by physical and instinctual impulses. The questions an animal usually has to answer at any given time mostly consist of, "Am I hungry? Am I safe? Am I in danger? Am I bored? Could I be mating?" Ethics and morality do not enter into this. A bear will not wonder about the ethical ramifications of eating a smaller animal. Its thoughts, I can only assume, center around satiating its hunger and finding an animal small enough to kill but large enough to satisfy.
That mankind can ponder the morality of animal life is a sign of its superiority. And part of what sets human life above animals is that we recognize this capacity in others. When we consider our actions, we do not account only for how it affects us. We also account for how it affects those around us. Any number of specific examples could be cited to contradict this, but it is the case in general.
I could write more. The point is that mankind's intelligence sets it apart from the animal kingdom, and it is the value of this capacity, and also a function of it, that sets the value of a human life above that of an animal.