Harold Cook is a rare thing among historians: a scholar who has written seminal books on large topics that not only incited much interest when they first appeared, but have also stood the test of time. In particular, his work on the history of 17th-century medicine and its connections to science, published in the 1980s, remains the first port of call for anyone interested in the subject. Nor has he rested on his laurels, for while that work focused on England, his interests since then have shifted elsewhere, primarily to the gloriously complicated, intertwining medical, scientific and commercial spheres of the early modern Dutch Republic.

Now his eyes have fallen on the pan-European peregrinations of René Descartes before the Frenchman became a famous philosopher. The focus is on the years 1615 to 1628 – that is to say, the very poorly documented period between Descartes’s departure from the Jesuit College of La Flèche, aged eighteen, and his move to the Netherlands in 1628. This period saw him become a gentleman soldier, first in the Netherlands – where he learned military engineering – and then in central Europe, where he served in the Bavarian army, possibly participating in the Battle of the White Mountain. In Hungary, the death of his commanding officer confirmed his disgust for warfare, and so he travelled back to France, at one point supposedly foiling a robbery and murder plot by the sailors who were transporting him.

His activities during the rest of the 1620s are even harder to piece together. He returned to his family in Poitou to look after the estate of his maternal grandmother; he also went to Italy, seemingly to try to become a senior army officer, but also to make