Leonardo da Vinci wrote backward (mirror writing) because he didn’t want others stealing his ideas. Writes Da Vinci biographer Rachel A. Koestler-Grack:
“The observations in his notebooks were written in such a way that they could be read only by holding the books up to a mirror.”
But did a genius who combined art and science so brilliantly really need to hide his work? Perhaps it was practical: as a lefty, he didn’t want to smudge the ink. As a contrarian, Da Vinci also strived to be different. As blogger Walker’s Chapters writes:
“Do you really think that a man as clever as Leonardo thought it was a good way to prevent people from reading his notes? This man, this genius, if he truly wanted to make his notes readable only to himself, he would’ve invented an entirely new language for this purpose. We’re talking about a dude who conceptualized parachutes even before helicopters were a thing.”