St. Jerome in the Wilderness

An unfinished Leonardo — and that is precisely why it's a revelation. You see the underpainting: the skull-like anatomy of Jerome's shoulder, the muscular twist of the neck (Leonardo dissected corpses to get it right), the lion barely sketched at the saint's feet. The face is finished enough to feel the psychological weight of penance. The painting was famously cut into pieces, one used as a stool cover in a shoemaker's shop, before being reassembled in the 19th century — you can still see the seam.