Spot's Meow wrote:
Emotion. I think that's really what it boils down to. You may be physically or mentally incapacitated, but you are still human because you still feel.
But animals have emotions. Our pets love us (and each other), elephants grieve for their dead, some animals even come to the aid of members of other species (dolphins being the most obvious example, but there are many others). What makes Humans different is that we can contemplate our emotions.