Let me make it clear that I'm not saying we should use intelligence purely to pick and choose. I was arguing that suffering isn't equal across all species merely because they have neurons, a point made by a previous poster. I'm personally OK with killing animals to eat them, or wear their fur for that matter as long as they're kept and killed well. I've kept animals myself and looked after them. They had a fine life and died better than most people I know. But intelligence is a factor in suffering and I'm willing to bet you'd feel better about killing a fly than than a chimp.
It's true that a retarded person or a baby wouldn't have the sense of threat at all and could be killed as humanely as a farm animal. And of course we do use this fact in abortion and turning off life support machines of coma victims etc.
Our little club is humans. Humans are our species. We have higher standards for ourselves. I don't think that counts as arbitrary. it's hardly on a whim or at random. our entire society is based on valuing other humans and empathising with them and we don't apply these standards to animals.
When you start arguing that animals should have the same position as humans you lose me.