Full Idea
If pain is a functional state, it cannot be a brain state, because creatures without brains could realise the same Turing machine as creatures with brains.
Gist of Idea
Could a creature without a brain be in the right functional state for pain?
Source
Ned Block (Troubles with Functionalism [1978], p. 70)
Book Reference
'The Philosophy of Mind', ed/tr. Beakley,B /Ludlow P [MIT 1992], p.70
A Reaction
This strikes me as being a poorly grounded claim. There may be some hypothetical world where brainless creatures implement all our functions, but from here brains look the only plausible option.