Full Idea
The physicalist should not retreat to causal supervenience but should stick with identity. This means we will have to accept that a Martian and I (when in pain) are not in the same phenomenal state.
Clarification
'Martians' are assumed to constructed in a different physical way from ourselves
Gist of Idea
If physicalists stick with identity (not supervenience), Martian pain will not be like ours
Source
John Perry (Knowledge, Possibility and Consciousness [2001], §4.3)
Book Reference
Perry,John: 'Knowledge, Possibility and Consciousness' [MIT 2001], p.88
A Reaction
We naturally presume that frogs feel pain as we do, but many different phenomenal states could lead to the same behavioural end. Only an unpleasant feeling is required. A foul smell would do. Frogs could function with inverted qualia, too.