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
Vasari's account of the "Mona Lisa" comes from his biography of Leonardo published in 1550, 31 years after the artist's death. It has long been the best-known source of information on the provenance of the work and identity of the sitter. Leonardo's assistant Salaì, at his death in 1524, owned a portrait which in his personal papers was named "la Gioconda", a painting bequeathed to him by Leonardo.