One concept I always found interesting is the idea of personhood: the idea that a creature with certain intelligent and, probably more important, emotional skills has a 'special' status in the animal kingdom. A lot of people used human and person interchangeably, even though they're not the same thing. While many humans are people, not all people are humans.

Just to give an example which most people can agree on: A human in a vegetative state is generally not considered a person. Sure, they are still human and they are alive. Yet that 'soul' or 'being' is missing. All that is left is an empty shell that is used to be a person.

So the question here is at what point can a human, or any creature for that matter, be considered a person? I always thought that some animals act more like a 'person' than human, though this is more due to a human either being severely mentally handicapped or being unable to care for nobody but themselves, just as a sadistic serial killer.