Pain is not a respecter of intelligence. We tend to assume that the stupidest individual we know is capable of feeling as much pain as we do. We don't tend to say:
"Well, Agnes is pretty dumb you know. Are we really sure that she felt pain when that truck hit her? Okay. I know she flew 25 feet in the air, let out a blood curdling scream and then thrashed about for ten minutes. But was that just a motor response? Was she really feeling it?"