there does not exist a prediction rule that takes a fairly detailed description of a being's* behavior in some task requiring intelligence and outputs a high accuracy classification.

To look for one presumes, as does Dallas, that the division between entities with rights and entities without is to be drawn in terms of current mental capability. Someone in dreamless sleep currently has less mental capability than an awake baby, or, I guess, a soon-to-be-born one. The argument for turning off Terri Schiavo's life support was that she had no possibility of regaining any mental function, not that she currently had none.