What is the central premise of the Muʿtazilī objection to charging the incapable?

Obligation Beyond Capacity: Possibility, Ability, and Moral Judgment

• Nonoccurrence establishes the intrinsic impossibility of conceiving the command
• Every kind of incapacity is identical to intrinsic contradiction
• Demanding an unperformable act is rationally objectionable and thus impermissible
• There is no basic difference between a challenge and a religious duty