6. Question 4 is based on this passage.
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 sec, 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.
4. The conclusion is properly drawn if which of the following is assumed?