Leonardo was a one-man corporation of brilliant ideas

Arriving at the last page of Charles Nicholl's enthralling, immensely detailed life of Leonardo da Vinci, I felt a strange mix of substantiality and uncertainty. So many of the Italian maestro's "flights of the mind," to use Nicholl's evocative subtitle, trailed off into the distance or ended in failure - including, apparently, an attempt at human flight. Leonardo was driven by fresh concepts, fascinated observations, renewed efforts. His astonishing diversity of interests never flagged. He was forever curious, inventive, investigative, and unquestionably painstaking. Yet phrases like "it came to nothing" recur throughout this book.

Completing a project was never Leonardo's strong point. Sometimes he was technically too experimental. Even in the case of the highly finished "Mona Lisa," he was unwilling to stop trying to perfect her.

Of course, incompletion was also the result of a career that forced him to dodge from patron to patron, from Florence under the Medicis to Milan under the Sforzas to Rome under the Medicis again and finally to France under François I. The times were restless, to say the least, but so was he.

His scattered fire may also have resulted from an uncertainty about his multiple gifts. For instance, he recommended himself to one patron as an inventor of war machines, mentioning his skill as a painter almost as an afterthought. In his last years, comfortably housed and given a generous pension by the French king, it was his knowledge and wisdom that were valued.

Nicholl concludes that it is his "writings and drawings which - perhaps even more than the paintings - take us directly into the life of Leonardo, as if they are themselves a kind of memory, cluttered with fragmentary records of the travails of his days, the secrets of his dreams, the flights of his mind."

As a refreshingly modern biographer, Nicholl takes this fragmentary character of Leonardo's work in stride. He offers clear insights into its many facets - the fascination with geometry, birds, horses, water, light, emblems, masques, grotesqueries, mountains, flight, anatomy, music, vortices, plants.... The list tends toward endlessness.

But the biographer is also up against extremely fragmentary documentation.

Nicholl brings us up to date with what is available today. He squeezes every drop from this material, and he's not shy about speculating.

For example, he logically assumes the young trainee Leonardo, in Verrocchio's workshop, helped with the engineering feat of erecting a two-ton copper orb on Florence's cathedral. He pictures Leonardo as one of the small figures poised atop Brunelleschi's dome. That image then suddenly becomes a fact. But was it? Nobody really knows.

On the other hand, Nicholl notes that a biographer needs to be more skeptical than romantic, and he smartly dismisses some of the highly romanticized claims about Leonardo that arose in the 19th century. At the same time, he cuts through much of the scholarly debate that today sometimes threatens to smother any cogent view of certain Leonardo issues.

He concludes, for instance, that Leonardo's late self-portrait, which is by no means certainly a self-portrait, is in fact the artist. And he argues that the subject of the Mona Lisa actually is Mona Lisa, a Florentine housewife, rather than accepting any of the accumulated and sometimes extravagant theories about her identity. What is the point, Nicholl asks, of seeking to solve a mystery if there is no mystery to solve? His section on this 501-year-old painting is perceptive and at times quite funny. She has, he is right, become virtually impossible to look at freshly or to take completely seriously.

This is a modern biography in its lack of veiling euphemisms when it comes to Leonardo's sexual life. But apart from discussing his homosexuality, Nicholl also posits the notion, certainly supported by some evidence, that the artist's entourage at one period included a female prostitute with whom he had relations.

Some of his last drawings bring together, disturbingly, certain dualities that preoccupied Leonardo throughout his life. He was never a religious man, and gender ambiguities undoubtedly interested him. There are figures that may be John the Baptist or Bacchus. Others may be male or female - or both.

He was not, at root, an artist willingly held back by conventions. Perhaps even these strange images were a kind of honesty - the integrity of the scientist-artist, objectively fascinated by the enigmatic world around and inside him.

• Christopher Andreae writes about art from his home in Glasgow, Scotland.