"The animal liberation movement is saying that where animals and humans have similar interests - we might take the interest in avoiding physical pain as an example, for it is an interest that humans clearly share with other animals - those interests are to be counted equally, with no automatic discount just because one of the beings is not human."

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...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.

— Peter Singer