Leonardo da Vinci was a great scientist as well a great artist. He possessed excellent memory and very lively imagination. His work shows integrity and belief in his self expressions.
As seen in his paintings, the subject of motherhood interested Leonardo. Thus, it can be assumed that he painted his mother. The Mona Lisa seems to be the only woman in his paintings (except the Virgin Mary and Saint Anne), who has a motherly expression on her face.
I am quite sure that the painting's subject is Leonardo's mother Caterina in a distant memory. She died in 1495. Lisa del Giocondo's job was to be the model only.
1.1 The eyes of Mona Lisa do not sparkle (usually the sparkle is manifested by a white spot) in spite of the direction of the source of light - which comes from the front. This indicates that Mona Lisa exists inside and outside of the spectator's world.
1.2 At the time that Leonardo painted the portrait of his mother, whom he adored, she was not among the living. This is the reason why Leonardo chose the setting of the Holy Land, as he imagined it, as the background to the portrait. (The Jordan River is painted to her right and the Sea of Galilee to her left). See: Cross and Yarn-Winder.
1.3 The idea is that she was alive in Leonardo's imagination.
2.1 This is similar to the background of Leonardo's paintings of the Virgin Mary, which also depict the same landscape of the Holy Land. Thus, Leonardo glorifies the Mona Lisa as the Virgin Mary. See: Leonardo glorifies Salai as Saint John the Baptist.
2.2 Leonardo kept the portrait with him wherever he traveled, until his death. Hence, the Mona Lisa was a significant woman in Leonardo's life.
2.3 Leonardo pictured his mother, who raised him until age five, in painting the Virgin Mary. He made an associative connection between these two women in his art.
2.4 So, Leonardo's mother was the only significant woman in Leonardo's life, hence deserved to be glorified as the Virgin Mary.
2.5 See also the painting Salvator Mundi by Leonardo. Leonardo and Salvator Mundi are very similar. See Leonardo's portrait by Francesco Melzi. Thus, Leonardo glorifies himself as Jesus. So, this indicates that Leonardo's mother was the only significant woman in Leonardo's life, hence deserved to be glorified as the Virgin Mary.
3.1 All the women in Leonardo's paintings who have a motherly expression on their face look at their child. In all of Leonardo's paintings (except Annunciation), the Virgin Mary looks at her son.
3.2 In this painting she looks at the painter.
3.3 The conclusion is that the painter is her son.
3.4 It appears as if her right hand is embracing her son who does not seem to be in her world.
4.1 During Italian Renaissance Leonardo's time, it was not acceptable to paint ordinary people, only important people or saints.
4.2 The woman appearing in the portrait known as Isabella d'Este (1499 - 1500) is actually Leonardo's mother Caterina in distant memory. The woman appearing in the portrait looks like the Mona Lisa and not like Isabella d'Este. See: Titian, Isabella d'Este, 1534 – 1536. They do not have the same forehead and lips.
4.3 This cartoon which has survived must have been drawn for an important work. See the cartoon in London, National Gallery. The only possible important work we are aware of is the Mona Lisa. See also the veil and the upper dress cut, rounded and not in straight lines.
4.4 Thus, Leonardo glorifies his mother as a respectable woman.
5. Finally, it is possible that Leonardo encoded the letters C and L (Caterina and Leonardo), in his special reverse style, in Mona Lisa's embroidery on her dress.
I reached these conclusions during my research about Leonardo's art.
