"...the nervous systems of other animals were not artificially constructed - as a robot might be artificially constructed - to mimic the pain behavior of humans. A capacity to feel pain obviously enhances a species' prospects of survival...it is surely unreasonable to suppose that nervous systems that are virtually identical physiologically, have a common origin and a common evolutionary function, and result in similar forms of behavior in similar circumstances should actually operate in an entirely different manner on the level of subjective feelings."