Whether the Eighth Amendment prohibits the execution of a prisoner who is insane.
Holding
Yes. The Eighth Amendment forbids a State from carrying out a death sentence against a prisoner who is insane.
Reasoning
The prohibition had deep roots in English common law. Blackstone, Coke, and other authorities treated execution of an insane person as cruel, inhuman, and purposeless. Although commentators offered different justifications for the rule, they agreed on the rule itself: execution must be stayed when a condemned person loses sanity.
The Eighth Amendment at least incorporates punishments regarded as cruel and unusual when the Bill of Rights was adopted. It also reflects evolving standards of decency. Both sources pointed in the same direction, because no State permitted the execution of the insane and the common-law prohibition had been carried into American law.
Executing a person who cannot comprehend why the State has condemned him fails to serve central purposes of capital punishment. It has little retributive value, offers no meaningful example to deter others, and offends the dignity and humanity protected by the Eighth Amendment.