Though it's human to err, admitting it doesn't come as naturally to the species - much less hiring someone to ferret out our mistakes. But that's exactly what the best government agencies do, setting up inspectors general, internal affairs departments, and the like to review and reconsider their work.

Philadelphia District Attorney Seth Williams deserves credit for his newly announced plan to correct some of his office's mistakes, which can have the serious consequences of putting innocent people in jail and letting guilty people go unpunished.

Williams this week announced the creation of a unit dedicated to investigating wrongful convictions, to be headed by Assistant District Attorney Mark Gilson, a veteran homicide prosecutor. One of Gilson's best-known prosecutions, for the 2000 murder of seven people in a crack house, was hampered by a wrongful arrest before it was successfully tried.

Some skepticism is warranted as to the willingness of a longtime prosecutor appointed by the district attorney to challenge his colleagues' convictions. Indeed, Williams' office has until recently shown plenty of resistance to the idea of reevaluating prosecutions, even in the face of serious questions.

Nevertheless, the Pennsylvania Innocence Project's legal director, Marissa Bluestine, was on hand to praise the formation of the Conviction Review Unit during a news conference this week. Her group has asked the District Attorney's Office to reopen 10 cases over the past three years. Among them is the conviction of two men in the 1995 slaying of a businessman based on evidence that a judge has called "extremely weak." The new unit will be measured by its ability to address such cases.

Across the country, an increasing number of convictions are being reversed not over the objections of prosecutors, but with their cooperation. Prosecutors' offices in New York City, Dallas, and the San Francisco area have established units to review problematic convictions and in some cases take the initiative in undoing them.

More than 300 convictions nationwide have been overturned by DNA evidence, according to the Innocence Project. Those exonerated had served an average of more than 13 years in prison, and more than 30 had served time on death row or faced the possibility of a death sentence.

The continued existence of the death penalty in Pennsylvania is of course at logical odds with the idea of prosecutorial fallibility. Now that the district attorney has acknowledged the latter, perhaps he will also reconsider his support for that thoroughly irreversible form of punishment.