Full Idea
According to functionalism, a system might have the behaviouristic input-output relations, yet not desire something, as this requires internal states with certain causal relations.
Gist of Idea
In functionalism, desires are internal states with causal relations
Source
Ned Block (Troubles with Functionalism [1978], p. 69)
Book Reference
'The Philosophy of Mind', ed/tr. Beakley,B /Ludlow P [MIT 1992], p.69
A Reaction
Such a system might be Putnam's 'superactor', who only behaves as if he desires something. Of course, the internal states might need more than just 'causal relations'.