Troy Davis died by lethal injection Wednesday in Georgia, 23 years after the murder of a police officer, a crime Davis said he did not commit. Death penalty supporters argue that Davis had more than ample opportunity to demonstrate his innocence; the Georgia Board of Pardons and Paroles rejected his request for clemency in the case. Which leaves us with the unsettling question: What if he really was innocent, and they were wrong?

Troy Davis died by lethal injection Wednesday in Georgia, 23 years after the murder of a police officer, a crime Davis said he did not commit. Death penalty supporters argue that Davis had more than ample opportunity to demonstrate his innocence; the Georgia Board of Pardons and Paroles rejected his request for clemency in the case. Which leaves us with the unsettling question: What if he really was innocent, and they were wrong?

A jury convicted Davis, 42, of shooting and killing off-duty Savannah, Ga., police officer Mark McPhail in 1989 outside a Burger King restaurant as he went to the aid of a homeless man who was being beaten.

Since the trial, seven of nine witnesses changed or recanted their testimony. Some said they were coerced by police to testify, and some said another man — also one of the nine witnesses — was the real killer.

No gun was ever found.

The death penalty has long been controversial in this country, especially since most developed nations ended the practice. The arguments for and against its use are well-known.

Supporters say the ultimate crime demands the ultimate punishment. Executing murderers, they argue, will deter others from committing murder, and will ensure a convicted murderer can never be released or escape from prison and kill again.

Supporters also argue that survivors of murder victims deserve to see the person responsible pay the ultimate price.

Some couch their support in biblical terms, referring to the Old Testament "eye for an eye" scripture that says punishment should fit the crime.

All of these points can be debated, and any of them might make a compelling argument — if one assumes the justice system never makes mistakes.

And yet we know beyond a shadow of a doubt that defendants have been wrongly sentenced to die in this country. In some cases their lives have been spared by DNA evidence that proves they were not guilty.

It is virtually certain that at least some defendants have been executed for crimes they did not commit.

There was no DNA evidence in Troy Davis' case. His life hinged on the testimony of witnesses whose stories changed over the years. We cannot know for certain that he was innocent, as he claimed. But we also cannot know for certain that he was guilty.

A person sentenced to prison who is later determined to be innocent can be freed. An execution cannot be undone.

A society that cannot guarantee its legal system is infallible has no business killing people who might be innocent.