Leonardo da Vinci: the Flights of the Mind by Charles Nicholl<br/> Leonardo by Martin Kemp

Numerous interpretations have been given of the Mona Lisa's mysterious smile. George Sand, rejecting the notion of seduction, detected in it cold malice. Conflicting views merely reinforce her enigma, while a similar frustration accompanies any attempt to learn more about Leonardo da Vinci's life.

Information on him is elusive, ambiguous and in places contradictory. A great cloud of fame surrounds his name but within it are large pockets of obscurity. He is also now very distant from us in time, and still locked, with Michelangelo and Raphael, in that great triumvirate that shaped the High Renaissance, its achievement casting a centuries-long shadow over subsequent art. How should a biographer, tiptoeing around the pedestal of this great colossus, begin? Charles Nicholl snags our attention with cooling soup.

This tiny detail is revealing about Nicholl's approach to biography. While studying a page of geometrical notes by Leonardo in the British Library, he noticed that the writing suddenly ends with an abrupt "etcetera". The following five words explain that the writer had to break off "because the soup is getting cold": a postscript Nicholl relished. "Simple, daily humanity", he remarks, intrudes into "the dry abstractions of his geometrical studies".

This glimpse into everyday reality offers a heartening reminder that the life of this extraordinary artist constantly intersected with ordinariness. It is at these moments, Nicholl argues, that the biographer - "that emissary sent out from the ordinary world" - can make contact with his or her subject.

This may be a trick of the trade, but it is one reason why Nicholl's book is so engaging. Another is the sheer fascination of his subject. From his obscure beginnings in a Tuscan backwater, as the illegitimate child of a notary, Leonardo became one of the most sought-after celebrities of his day, spending 30 years in courtly employment and ending his career in French royal service.

Initially apprenticed to Andrea Verrocchio, he became, in 1472 at the age of 20, a member of the Florentine painters' confraternity. But his curiosity and intellectual energies soon spilled into other areas.

In his famous letter to Ludovico Sforza in Milan, seeking employment, he offered a range of skills. He made most of his ability to contrive a battery of military hardware. Almost as an afterthought, his letter ends: "In time of peace I believe I can give complete satisfaction, equal to any other man, in architecture... and in guiding water from one place to another. Also I can undertake sculpture in marble, bronze or clay, and in painting I can do everything that it is possible to do, as well as any other man who ever he may be."

It remains a puzzle why Leonardo wished to leave Florence under the Medicis for the culturally inferior Milan and the patronage of the relatively uncouth Sforzas. Nicholl shows the received view - that he was a protégé of the Medicis - has shaky foundations. Here, and elsewhere, he excavates the fragments of knowledge that we have about Leonardo and reactivates them with his detailed knowledge of the political, social and intellectual context. Whether discussing Leonardo's fellow artists, his pupils, patrons, literary chums or philosophical gurus, or just walking us through a landscape, a city or castle, he makes vivid the past.

A humane understanding plays through his lively, fluent prose. It occasionally leads him to make suppositions that do not always convince, in part owing to the sparsity of facts. But, overall, this biography is a hugely impressive feat of historical and imaginative empathy.

Nicholl's attentiveness to great and small things matches the contents of Leonardo's notebooks. According to an eye-witness account in Milan, he always kept one of these small books hanging from his belt. In these he would jot down jokes, doodles, snatches of poetry, shopping lists, accounts, names and addresses, half-sentences as well as carefully elaborated scientific treatises on a wide range of subjects: anatomy, architecture, botany, costume design, civil and military engineering, hydrography, mechanics, optics, philosophy and stage design.

These notebooks reveal his speculative brilliance. In one passage, quoted by Nicholl, he arrives at the idea of human flight: "See how the beating of its wings against the air supports a heavy eagle in the highly rarefied air... Observe also how the air in motion over the sea fills the swelling sails and drives heavily laden ships... A man with wings large enough, and duly attached, might learn to overcome the resistance of the air."

In Martin Kemp's opinion, these notebooks are "utterly exceptional". His small book on Leonardo uncovers the constructs that underpin the surface diversities of this artist's thought. He shows how Leonardo worked by analogy. Wanting to understand the circulation of the blood and the effects of age on the arteries, he observed the silting up of rivers.

The body itself was viewed as the world in miniature, exhibiting principles of organisation - the fitting of form to function - that operated in nature. Through Kemp, the reader is introduced to the great intellectual and visual edifice that Leonardo erected in his notebooks. We are also reminded of just how many of his assumptions or conclusions have become part of the science or technology of the modern age.

Kemp is a world authority on Leonardo and is able to distill in relatively few pages knowledge acquired while lecturing, researching and writing over many years. Certain key issues and specific works are used to illuminate a number of Leonardo's core beliefs. There is also a useful chapter in which the material circumstances of Leonardo's career is offered as a sober counterweight to the accumulation of legend.

We learn in the introduction that the book was written in 11 days, while Kemp was staying at a holiday villa in Tuscany that formerly belonged to the Gherardini family. One of them, Lisa, sat for Leonardo and became the subject of the most famous painting in the world. Like Nicholl, Kemp now and then alludes wryly to the many ironies spawned by Leonardo's unwieldy fame.

More important, his short study underlines the predominantly visual intelligence behind Leonardo's thinking. For him, seeing was understanding, and every act of looking simultaneously an act of analysis. Few of us would dispute Kemp's claim that "no one ever looked more intensely".

Frances Spalding's life of Gwen Raverat is published by Harvill