perhaps: given a poorly defined domain construct a decision theory that is as close to optimal (given the goal of some future sensory inputs) as your sensory information about the domain allows.

This doesn't give one a rigorous way to quantify intelligence but does allow us to qualify it (ordinal scale) by making statements about how close or far away various decisions are from optimal. Otherwise I can't seem to fold decisions about how much time to spend trying to more rigorously define the domain into the general definition.