A parable from Roger Scruton (and my primary school head)

I originally wrote about this story in March 2009:

He also told with great feeling an apocryphal story concerning the composition of Leonardo's Last Supper, which, in this version of event, the artist composed over many decades, constantly searching the streets and alleyways of Milan for the ideal types upon whom to model the twelve apostles, and having begun with the beautiful and innocent face of a young man whose expression seemed to capture all the grace, dignity and tender compassion of Jesus. 

After years of labour the apostles had all been assembled, representing in their carefully differentiated expressions the fine gradations of hope, resolution, weakness and despair.
Only one remained and that was Judas, whose baseness no citizen of Milan seemed to wear on his face, and to whom Leonardo began to despair of giving the absolute lifelikeness that was vital to his conception. 

At last, in a mean alleyway, a dark figure, engaged in some whispered transaction, caught the painter's eye. Recognising in those fear-filled, treacherous glances the lineaments of Judas, Leonardo enticed him to the cenaculo with a gift of silver. 

The figure, shifty, suspicious and huddled into himself, is pushed into a corner and told to sit. Looking up at last, and recognising the painter and the tools of his trade, he says, "You have painted me before." 

"Have I?" asks the startled painter. "When?" 

"Oh, a long time ago." 

"And for what purpose?" 

Judas turns to the nearly completed fresco that is taking shape above them. 

"There I am, he says, and points to Christ.