Bluematter.

I am an innocent man, and something very wrong is taking place tonight. May God bless you all. I am ready.

This is from the last statement of L T Herrera, from a long, comprehensive list of last statements preserved by the Texas Department of Criminal Justice. (via Pharyngula)

Excerpts from the opinion of the Supreme Court, delivered by Chief Justice Rehnquist, including a description of the case:

Petitioner Leonel Torres Herrera was convicted of capital murder and sentenced to death in January 1982. He unsuccessfully challenged the conviction on direct appeal and state collateral proceedings in the Texas state courts, and in a federal habeas petition. In February 1992--10 years after his conviction--he urged in a second federal habeas petition that he was "actually innocent" of the murder for which he was sentenced to death, and that the Eighth Amendment's prohibition against cruel and unusual punishment and the Fourteenth Amendment's guarantee of due process of law therefore forbid his execution. He supported this claim with affidavits tending to show that his now dead brother, rather than he, had been the perpetrator of the crime. Petitioner urges us to hold that this showing of innocence entitles him to relief in this federal habeas proceeding.[Emphasis DC] We hold that it does not.

[...] In any system of criminal justice, "innocence" or "guilt" must be determined in some sort of a judicial proceeding. Petitioner's showing of innocence, and indeed his constitutional claim for relief based upon that showing, must be evaluated in the light of the previous proceedings in this case, which have stretched over a span of 10 years.

Once a defendant has been afforded a fair trial and convicted of the offense for which he was charged, the presumption of innocence disappears. [...] ("The purpose of the trial stage from the State's point of view is to convert a criminal defendant from a person presumed innocent to one found guilty beyond a reasonable doubt"). In the eyes of the law, petitioner does not come before the Court as one who is "innocent," but on the contrary as one who has been convicted by due process of law of two brutal murders.

And excerpts from the dissent, delivered by Justice Blackmun:

We really are being asked to decide whether the Constitution forbids the execution of a person who has been validly convicted and sentenced but who, nonetheless, can prove his innocence with newly discovered evidence. Despite the State of Texas' astonishing protestation to the contrary [...] I do not see how the answer can be anything but "yes."

The protection of the Eighth Amendment does not end once a defendant has been validly convicted and sentenced. In Johnson v. Mississippi, 486 U.S. 578 (1988), the petitioner had been convicted of murder and sentenced to death on the basis of three aggravating circumstances. One of those circumstances was that he previously had been convicted of a violent felony in the State of New York. After Johnson had been sentenced to death, the New York Court of Appeals reversed his prior conviction. Although there was no question that the prior conviction was valid at the time of Johnson's sentencing, this Court held that the Eighth Amendment required review of the sentence because "the jury was allowed to consider evidence that has been revealed to be materially inaccurate." [...]

Just as an execution without adequate safeguards is unacceptable, so too is an execution when the condemned prisoner can prove that he is innocent. The execution of a person who can show that he is innocent comes perilously close to simple murder.

The opinion, and to a lesser extent the dissent, do a horrible job of expaining and addressing the issue at hand.

Society has a certain 'model' ('judicial proceedings') it uses to determine whether a person in guilty or innocent. The issue before the Court is really very simple: does the expected gain in the predictive power of the model as a result of utilising the previously unavailable information justify the cost - in a wide sense - of a re-trial (i.e. re-running the model)? Both the opinion and the dissent touch on this in an incoherent manner, and treat these considerations as peripheral.

Chief Justice Renquist's thesis basically consists of stating that re-trials are costly so we need to take into account the output of previous trials even when these did not utilise all available information ('once a defendant has been afforded a fair trial and convicted of the offense for which he was charged, the presumption of innocence disappears.') while Justice Blackmun is mainly concerned with asserting that utilising more information would be beneficial. In the end, there is no systematic attempt at stacking one against the other.

When will courts start thinking and arguing clearly? The wonderful lessons of economics - define a clear question, explicitly state your assumptions, reach a robust conclusion - must make their way to the hallowed halls of justice, where they will find a natural, welcoming home.