`We will never understand Leonardo if we think of him as a modernist among the Medicis'
Leonardo da Vinci wrestled with the belief that fossils provided a key to the earth's structure. His findings on fossils were enshrined in the `Leicester Codex', so called because it was the property of the Earl of Leicester for more than 250 years. Professor Stephen Jay Gould contends that the `Codex' as a whole is indeed a proposal for the structure of the earth, and is also a crucial insight into Leonardo's artistic and spiritual beliefs.

The irony of Leonardo's notebooks is that by the time they were published a couple of centuries later, most of his wonderful observations had already been made again. So Leonardo comes across as a spaceman, a man out of his time. The myth continues that Leonardo stood alone and above because he combined his unparalleled genius with a thoroughly modern methodology based on close observation and clever experiment.

I think that this conventional view couldn't be more wrong in its general approach to the history of knowledge, or more stultifying in our quest to understand the most fascinating man of our intellectual past. We'll never understand him if we insist on reading him as a modernist among the Medicis, a futurist at the court of Francis the First. Leonardo operates in the context of his time. To understand him we must chronicle and respect the medieval sources and character of his thought.

I'm going to begin by acknowledging the truly prescient character of the observations, but what alternative account of fossils was Leonardo trying to disprove with these observations, and secondly, what theory of the earth was he trying to support?

I have a favourite line from Darwin, from an 1863 letter: "How can anyone not see that all observation must be for or against some view if it is to be of any service?" Leonardo's keen observations do seem to emit a wonderful whiff of modernity, but he recorded all his information explicitly to confute the two major interpretations of fossils current in his day, which had been proposed to resolve a problem. Namely, if fossil shells were the remains of marine organisms, how did they get entombed in strata that lie in mountains thousands of feet above present sea levels?

The first idea that he wants to ridicule is that the fossils were transported by the high waters and violent currents of Noah's flood - an idea that remained standard until the 18th century.

He deals even more contemptuously with neo-platonic versions of the theory that fossils are not remnants of ancient organisms at all, but manifestations of some plastic force within the rocks or some emanations from the stars capable of precisely mimicking a living creature in order to illustrate the symbolic harmony among realms of nature. Because if the fossils really belong to the mineral kingdom, then their position on the tops of mountains ceases to be anomalous: they're not real organisms.

This was a real issue, and the confutation of it was a major event in 17th-century paleontology.

Just as Leonardo made his astute observations to refute prevailing theories of fossils, he also words his interpretations to support his own favourite theory. On this point, he could not have been more squarely Renaissance or late medieval, more firmly attached to his own time and concerns, and not to ours.

Why does he devote so much apparently subsidiary space to the nature of fossils? The key to the problem lies in water, and in Leonardo's almost heroic struggle to overcome the essential difficulty in validating his crucial analogy of the earth's macrocosm to the body's microcosm. Both are made of the four elements: earth, air, fire and water. But the human body saves itself by circulating these elements, particularly by maintaining some mechanism for permitting water to rise from the legs to the head. The analogy of microcosm to macrocosm can work only if the earth also possesses a comparable mechanism. He knew water moved up - somehow - because it came out as springs at the top of the mountain.

Now, here's the central irony. He never did solve the problem that was the main subject of the Leicester Codex: he never found a satisfactory mechanism to account for the upward motion of water. However (and this is a vital point that has usually been missed), Leonardo did succeed in his quest to find a mechanism for the upward motion of the other heavy element: earth. The fossils on the mountains provide the observational truth that the earth can rise.

His argument is that the earth's structure is not homogeneous. It's a marbled mass of rocks and internal canyons and of waters - one hemisphere of the earth, as Leonardo saw it, is heavier than the other. The earth has to balance itself as a see-saw: if you're heavier, you have to move yourself closer to the centre. Therefore, the heavier hemisphere has to move towards the centre and the lighter hemisphere has to move away, to keep the balance.

Leonardo thinks that every once in a while a piece from the earth's interior is eroded - in this illustration, from the upper hemisphere - and falls into the centre and drapes itself around the centre of gravity. The bottom hemisphere gets heavier and moves in towards the centre, and the upper hemisphere rises, and the strata that contained the fossils underwater are pushed up to form mountains. What's the proof that this is true? Again, the fossils on the mountains.

Now we can grasp the central importance of Leonardo's paleontological observations in the Leicester Codex. He features this subject in order to validate the cherished centrepiece of his pre-modern world view: the earth as living, self-sustaining organism. He requires above all a general device to make the heavy elements, earth and water, move upwards against their natural inclination so that the earth can sustain itself like a living body. He failed to find such a mechanism for the chief subject of the Leicester Codex, water, and his lack of resolution caused him great frustration, but he succeeded, as he thought, for the even heavier element of earth. But he needed evidence that land did in fact rise.

Thus, Leonardo made his superb observations in order to validate his lovely but antiquated view of a meaningful and precise unity between the human body's microcosm and the earth's macrocosm. Leonardo: a truly brilliant observer. No spaceman, but a citizen of his own instructive and fascinating time.

Professor Stephen Jay Gould: `Leonardo's "Modern" Observations on Fossils: The Medieval Context and Rationale'. Part of the Last Word lunch-time lecture series at the Royal Geographical Society