Isleworth Mona Lisa
According to Leonardo's early biographer Giorgio Vasari, Leonardo had started to paint "Mona Lisa" in 1503, but "left it unfinished". However, a fully finished painting of a "certain Florentine lady" surfaced again in 1517, shortly before Leonardo's death and in his private possession. The latter painting almost certainly is the one that now hangs in the Louvre. Based on this contradiction, supporters of the authenticity of the "Isleworth Mona Lisa" claim it is the unfinished "Mona Lisa", made at least partially by Leonardo, and the Louvre "Mona Lisa" a later version of it, made by Leonardo for his own use.
Mona Lisa
However, this portrait does not fit with the description of the painting in the historical records: Both Vasari and Gian Paolo Lomazzo describe the subject as smiling; the subject in Cotte's portrait displays no smile. In addition, the portrait lacks the flanking columns drawn by Raphael in his c.1504 sketch of "Mona Lisa". Moreover, Cotte admits that his reconstitution had been carried out only in support of his hypotheses and should not be considered a real painting; he stresses that the images never existed. Kemp is also adamant that Cotte's images in no way establish the existence of a separate underlying portrait.