X-ray examination of a recently discovered painting -- judged by some authorities to be a self-portrait by Vincent van Gogh -- revealed an underimage of a woman' s face  Either van Gogh or another painter covered the first painting with the portrait now seen on the surface of the canvas
Because the face of the woman in the underimage also appears on canvases van Gogh is known to have painted, the surface painting must be an authentic self-portrait by van Gogh
A painted canvas cannot be reliably attributed to an artist unless the authenticity of any underimage that painting might contain can be reliably attributed to the artist.