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Rh ,” “mental disorder,” and “psychological dysfunction.” 551 U. S., at 936, 959, 960; see Ford, 477 U. S., at 408–409, n. 2 (referring to prisoners with “mental illness”). And most important, Panetti framed its test, as just described, in a way utterly indifferent to a prisoner’s specific mental illness. The Panetti standard concerns, once again, not the diagnosis of such illness, but a consequence–to wit, the prisoner’s inability to rationally understand his punishment.

And here too, the key justifications Ford and Panetti offered for the Eighth Amendment’s bar confirm our conclusion about its reach. As described above, those decisions stated that an execution lacks retributive purpose when a mentally ill prisoner cannot understand the societal judgment underlying his sentence. See Panetti, 551 U. S., at 958–959; Ford, 477 U. S., at 409; supra, at 2–3. And they indicated that an execution offends morality in the same circumstance. See 551 U. S., at 958, 960; 477 U. S., at 409; supra, at 2–3. Both rationales for the constitutional bar thus hinge (just as the Panetti standard deriving from them does) on the prisoner’s “[in]comprehension of why he has been singled out” to die. 477 U. S., at 409; see supra, at 2–3. Or said otherwise, if and when that failure of understanding is present, the rationales kick in–irrespective of whether one disease or another (say, psychotic delusions or dementia) is to blame.

In evaluating competency to be executed, a judge must therefore look beyond any given diagnosis to a downstream consequence. As Ford and Panetti recognized, a delusional disorder can be of such severity–can “so impair the prisoner’s concept of reality”–that someone in its thrall will be unable “to come to grips with” the punishment’s meaning. Panetti, 551 U. S., at 958; Ford, 477 U. S., at 409. But delusions come in many shapes and sizes, and not all will interfere with the understanding that the Eighth Amendment requires. See Panetti, 551