Leading article: Private eye

Tate Modern has scored a great coup in unveiling a hitherto rarely glimpsed 1932 work by Picasso, Nude, Green Leaves and Bust. Portraying the artist's then mistress and muse, Marie-Thérèse Walter, and sold for £70m last May, its previous owners exhibited it only once – to mark the artist's 80th birthday in 1961.

To many people, the idea that a great work of art should be hidden away for so long is unsettling, indeed almost an affront. The Tate's decision to put this mysterious Picasso on show is welcome. But we should remember that the idea that all great art should be on public display is relatively new.

The principle gained force only after the French Revolution, when countless royal collections were seized in the name of "the people" and huge canvasses came into fashion, designed to fit big, new national galleries.

Until then, much of what was not designed to adorn a church roof or high altar was destined for the prince's, or cardinal's, studiolo – his private cabinet of jewels and curiosities.

Who knows how many forgotten or half-forgotten great works lie unseen behind the closed doors of secretive collectors? Many more, one hopes. Each time that a hidden masterpiece goes on display, it is as if the picture itself had been reborn.