A dissertation is a long work which argues a specific point-of-view (or thesis). Therefore, there are many different stands one can take on Leonardo da Vinci. (On a side note, one needs to focus a dissertation on a topic/stand which has not already become saturated by other writers.)
1. The contrast of nature and man in Leonardo da Vinci's works.
2. The difference between his drawings and his paintings.
3. Assumptions made regarding why it took so long for Leonardo da Vinci to complete his pieces.
4. Choose different mediums of Leonardo da Vinci's work and decide upon the similarities and differences between the group. Define his work as consistent or inconsistent regarding different mediums.