In this chapter, Isaacson explores Leonardo’s investigations into geology, hydrodynamics, and the interconnectedness of natural systems. Using the analogy of the human body (microcosm) and the earth (macrocosm), Leonardo initially believed that veins, blood, and breath had direct counterparts in rivers, oceans, and tides. This concept informed his Codex Leicester, a notebook containing his observations and theories about geology, water, and astronomy. He studied erosion, water flow, fossils, and mountain springs, conducting both physical and thought experiments. Ultimately, Leonardo revised his original analogy, concluding that mountain springs result from rainfall and evaporation rather than subterranean circulation. His discoveries about erosion, sedimentary layers, and fossils challenged biblical flood narratives and prefigured modern geology. He also accurately described atmospheric optics, such as why the sky is blue. The chapter underscores Leonardo’s evolving scientific rigor, observational brilliance, and capacity to challenge even his own cherished ideas.

In 1512, with Milan politically unstable, Leonardo relocated to the Villa Melzi, where he lived with his pupil Francesco Melzi and continued anatomical and geological studies. He drew architectural plans, dissected animals, and revised his Codex Leicester, but did not compile his studies into treatises.