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And he did this, as Mr. Graham-Dixon explains, by using common people - the poor, criminals, even prostitutes, as models for his saints, including the Virgin Mary.

And he painted them in scenes without clouds and cherubs and hosts of winged angels, but in everyday life where feet were often dirty, clothing threadbare and people worn by the cares of life and poverty.

To this day, as Mr. Graham-Dixon points out, Caravaggio’s greatest religious works - “The Seven Acts of Mercy,” for example, or “The Supper at Emmaus” - have lost none of their power to startle and fill with awe.

Mr. Graham-Dixon is first-rate on Caravaggio’s religious side. Where he fails is in dealing with the bad-boy side. For Caravaggio’s great works include not only deeply religious works, but also very secular ones, “Self-Portrait as Bacchus” and “The Cardsharps,” to name but two, which offer a vivid look at the other side of his life.

Thirteen years ago, Peter Robb, an Australian biographer of Caravaggio emphasized the artist’s bad-boy qualities. In Mr. Robb’s portrait - “M: The Man Who Became Caravaggio” (1998) - the painter is violent, ambitious, primarily homosexual and dissolute.

It is a no-warts-barred look at a man of genius that must have left some readers wondering how a man bent on self-destruction would have had the time or inclination to do the great work he did, whether religious or secular.

It’s Caravaggio’s wild side that Mr. Graham-Dixon seeks to play down in this new biography. This is a tamer man, and an artist devoted to his art.

Yet out of the mold Mr. Graham-Dixon puts him in, Caravaggio keeps breaking out, wild and ungovernable. Perhaps no book can explain this extraordinary painter, and his admirers will have to be satisfied with his two natures, neither of which seems in happy balance with the other.

• Stephen Goode wrote on art for Insight magazine from 1987 to 2004.