We have observed typical pain inducing behaviour, the body being pinched, we have observed typical behavioural consequences of pain in his writhing and groaning. This is after all no different from what we observe in our fellow humans. Their pain itself is never the direct object of our observation but always inferred from it. If we assert that human beings are in pain on the basis of pain-inducing events and of pain behaviour, then, on functional grounds, it seems equally plausible to assert that the Martian is in pain. However, when the Martian is closely examined under a brain scan, it is revealed that, unlike human beings, he has no nervous system but instead hydraulic system of fluid-filled cavities'.
Human brains serve a function which can be performed equally well by other mechanical systems.
Is it the case that ontology is insignificant in understanding a causal series. In the Martian thought experiment the hydraulic system performs a linking role between pain inducing events and the pain behaviour. There n -my be other alternatives but the scope is certainly not unrestricted. Presumably the hydraulic system works as a receiver and transmitter in the sequence of events, from input to output. But not everything could assume this role.