[1] The Court, in an opinion by Justice Marshall, reviewed the evolving standards of the Eighth Amendment to be those consistent with "the progress of a maturing society", therefore not tolerant of acts traditionally considered "savage and inhumane", as the execution of the mentally insane was in early English and American common law.

The Court reasoned that executing the insane did not serve any penological goals and that Florida's procedures for determining competency were inadequate.

Thus, the Court made a preliminary finding that the Eighth Amendment bars states from inflicting capital punishment upon insane persons.

[1] The Court then further addressed the procedural issues present in making a determination of insanity for Eighth Amendment concerns.

[1] Justice Rehnquist, in dissent, stated a belief that in common-law tradition, it was actually the executive branch that was the sole arbiter of decisions involving the sanity of prisoners sentenced to death.