[Sparrow]

How do we know anything feels pain apart from ourselves? So there may not be a human experience of pain.

In 'Animal Liberation', Singer says:
"Nearly all the external signs which lead us to infer pain in other humans can be seen in other species..." he then quotes from scientific reasearchers, e.g. the aptly named Lord Brain ('one of the most eminent neurologists of our time'):
"I personally can see no reason for conceding mind to my fellow men and denying it to animals...I at least cannot doubt that the interests and activities of animals are correlated with awareness and feeling in the same way as my own, and which may be, for aught I know, just as vivid."

"Every particle of factual evidence supports the contention that the higher mammalian vertebrates experience pain sensations at least as acute as our own." Richard Serjeant 'The Spectrum of Pain'.