Anyone Know This Problem ('The Hanging Judge')?

[SkinkHero]

This is from 101 Philosophical problems.

The hanging judge

Now Judge Dread had had many disagreeable people before him, but this one, who styled himself ‘the Philosopher’, despite never having studied the subject, had really annoyed him. Dread says:

‘I intend to teach you the value of honesty, prisoner. You have been found guilty of being a crook and a swindler and of repeatedly and systematically lying to the court to try to save your wretched skin. Well, justice has caught up with you now, my friend. The sentence of this court is…’ (here the Judge pauses for effect and dons a pair of black gloves and a little black hat) ‘…that you be taken from here to a place of execution and hanged by the neck until you are dead.

… BUT, as I am a magnanimous Judge, I shall give you one more opportunity to learn the value of truth. If, on the day of your execution, you sign a statement making one true declaration, the sentence will be commuted to ten years imprisonment. If, on the other hand, your statement is, in the view of the Chief Executioner, false, the sentence will be carried out immediately. And I warn you,’ Dread adds, seeing his words having no effect on the crook, ‘the Chief is a member of the Logical Positivist Executioners’ Club and will dismiss any metaphysical nonsense as false, so don’t try any of your tricks on her! There, now you have one day in which to make your choice!’

At this the jury applaud at the severity of the sentence and everyone in the courtroom looks at the defendant, pleased to see such a villain get a heavy sentence, coupled with the humiliating public true declaration. But, strangely, the Philosopher just smirks back as he is led away to Death Row.

The day of the execution arrives and the crook, beaming, signs a declaration which is handed to the Chief Executioner who reads it with growing bewilderment. Then, snarling, she crumples it up and orders the Philosopher be released, with no penalty whatsoever to be imposed.

What could the prisoner have said in the statement to have saved himself?