His ability to combine art and science, made iconic by his drawing of a perfectly proportioned man spread-eagle inside a circle and square, known as Vitruvian Man, made him history's most creative genius.
-- Walter Isaacson (Author) (Introduction)

Importance: This quote establishes Leonardo's Vitruvian Man as a symbol of his interdisciplinary genius as well as the importance of combining the fields of the arts and sciences. This interdisciplinary penchant is a major part of what establishes Leonardo as the quintessential Renaissance Man. In addition, the fact that several of his contemporaries were also studying and illustrating the Vitruvian Man positions him within the cultural, scientific, and artistic ethos of his day which sought to revitalize the works of Ancient scholars like Vitruvius.

In fact, Leonardo's genius was a human one, wrought by his own will and ambition. It did not come from being the divine recipient, like Newton or Einstein, of a mind with so much...
-- Walter Isaacson (Author) (Introduction)