THE Mona Lisa in the Louvre has been accepted for four centuries as the one, only, and original version of the famous portrait of Madonna Lisa Giocondo painted by Leonardo da Vinci. It is difficult to break down a tradition of such long standing, yet this is what is claimed to be done in the following pages. But in order to accomplish this, theories and arguments, no matter how strong and plausible they be, count as nothing unless substantiated by facts and direct contemporaneous evidence, and it is on these latter that the onus probandi lies. The fact that there are two Mona Lisas in existence to-day, both of superlative intrinsic merit, and both the work of Leonardo da Vinci, the one with a record of four centuries behind it, the other which has scarcely been heard of before and has only just emerged from obscurity, creates a Sphinx-like problem not easy to solve. The unknown Isleworth Mona Lisa can, however, afford to stand on her own merits and cast her enigmatic smile on those who taunt her with her lack of pedigree. But convinced of the genuineness of the Isleworth painting, and that upon the authority of the soundest of expert knowledge, I determined to solve the riddle. How I have succeeded I leave the reader to judge. As, however, this treatise is complex and discursive, I purpose here to give a short outline of its whole theory.