Leonardo's paintings, frescos, architecture and even warfare inventions were all incredible feats of engineering, skill and flare. His meticulous observations and thirst for knowledge based on first-hand study, allowed him to find techniques never seen before and hardly seen after.
Studying anatomy by dissecting corpses, he was able to translate his knowledge into his paintings and frescos, in the way limbs folded and muscles had to be shaded.
His studies in nature gave him inspiration for many inventions and architectural designs.
However, all the masterpieces of art and architecture and also all the inventions he put to practice are not for me what defined him as a genius. Outstanding and unique as all those things were, they were made and were delivered by an artist/architect/engineer of special talent. But because they were made they simply were beautiful, innovative yes, probably only possible by Leonardo alone.
Where I think his true genius lies is in those concepts which were never made because the material and the necessary technology was not available at the time.
Those particular inventions displayed his vision for the future. He was able to draw, maybe in a corner of a page full of other ideas, objects which would not have occurred to other contemporaries because his mind worked in a less restricted way. He was not just a painter, he was a painter because he was a scientist; he was an architect because he was a biologist, and so on. His mind would tap into many subjects, a true mirror into the Renaissance philosophy while incorporating all the disciplines into one person.
A classic example was his parachute, he had designed it and gave all the measurements. It was only finally tested last year, more than 500 years after its inception, and it worked: four wooden beams, a pyramid-shaped canvas, dropped by a hot air balloon.
Other examples were the ball bearings concepts and the bicycle; they all took centuries before being built but the vision was there, in the 15th century.
