And then he saw the light....Pierre Duhem (1861–1916) was a French physicist and historian of science. Before 1904 this famous scientists was quite certain that all of chemistry and physics, including mechanics, electricity, and magnetism, could be understood from thermodynamic first principles [law of matter and energy]. In this regard, saturated in the 'Enlightenment' and Atheist propaganda that no science was known before Voltaire, he regarded the Middle Ages as bereft of scientific advancement. Like many others who actually decided to study the subject he proved himself utterly wrong.

Duhem converted to the common-sense idea of what some call, 'continuity' and incremental change in scientific understanding. It was not dark for 1000 years, and then presto! the light shone and Galileo an earnest Catholic appeared. Copernicus and Newton both stated that they owed their achievements to the insights and labor of others. Some 500 medieval scientists in all manner of disciplines could be named.

It was Duhem who began to bring light and understanding to the subject of medieval science. In 1904, he found an unusual reference to a then-unknown medieval thinker, Jordanus de Nemore (d. 1260). From this small beginning Duhem invented the history of medieval science. Where Duhem's previous histories had been silent or negative about pre-1600 science in the Middle Ages, Les origines de la statique contained a number of chapters on the subject with one considering the impact of Jordanus de Nemore; another his followers; and a third argued about their influence on Leonardo de Vinci.

In his second volume of the same name, Duhem greatly extended his historical scope returning to the middle ages, spending four chapters on geostatics, including the work of Albert of Saxony in the fourteenth century. This investigation formed the basis of Duhem's best book on the subject of Middle Age science, Etudes sur Léonard de Vinci, and Le Système du monde, in which his thesis of the continuity of late medieval and early modern science is illustrated and quite proven.

From 1906 to 1913, Duhem studied the scientific notebooks of Leonardo de Vinci and published a series of essays uncovering de Vinci's medieval sources and their influences on the moderns. This was an important link in the continuity thesis. Da Vinci and his 'Renaissance' contemporaries were undoubtedly part of the Middle Ages and owed a great deal to what preceded them. Common sense would confirm this. In Etudes sur Léonard de Vinci Duhem's proposed the obvious and correct impression that Galileo's efforts were all preceded by a medieval heritage:

When we see the science of Galileo triumph over the stubborn Peripatetic philosophy of somebody like Cremonini, we believe, since we are ill-informed about the history of human thought, that we are witness to the victory of modern, young science over medieval philosophy, so obstinate in its mechanical repetition. In truth, we are contemplating the well-paved triumph of the science born at Paris during the fourteenth century over the doctrines of Aristotle and Averroes, restored into repute by the Italian Renaissance. (1917, 162; 1996, 193.)

Duhem presented Galilean dynamics as a continuous development out of medieval dynamics. We know of course, that this is entirely true, yet in the early 20^th century it was a heresy. He recovered the late medieval theory of impetus, tracing it from John Philoponus' criticism of Aristotle, to its maturity in the fourteenth century works of John Buridan and Nicole Oresme:

“The role that impetus played in Buridan's dynamics is exactly the one that Galileo attributed to impeto or momento, Descartes to ‘quantity of motion,’ and Leibniz finally to vis viva. So exact is this correspondence that, in order to exhibit Galileo's dynamics, Torricelli, in his Lezioni accademiche, often took up Buridan's reasons and almost his exact words” (1917, 163–62; 1996, 194).”

Duhem then sketched the extension of impetus theory from terrestrial dynamics to the motions of the heavens and earth. No one since the Christian era began had ever believed in a flat earth, and many Middle Age scientists were convinced that the earth was not the center of the universe. Gravity was known long before Galileo:

“Nicole Oresme attributed to the earth a natural impetus similar to the one Buridan attributed to the celestial orbs. In order to account for the vertical fall of weights, he allowed that one must compose this impetus by which the mobile rotates around the earth with the impetus engendered by weight. The principle he distinctly formulated was only obscurely indicated by Copernicus and merely repeated by Giordano Bruno. Galileo used geometry to derive the consequences of that principle, but without correcting the incorrect form of the law of inertia implied in it. (1917, 166; 1996, 196.)”

Duhem went further and linked Albert of Saxony [from the 14^th century] whose works were printed and reprinted during the sixteenth century, to Galileo. Duhem's key to understanding the transmission of medieval science was Galileo's use of the phrase Doctores Parisienses, which indicated the Parisian scholars Buridan and Oresme, amongst others. Duhem had rightly conjectured that Galileo had used George Lokert's biography of Albert of Saxony, along with the works of the Dominican Domingo de Soto (1906–13, III.582–83). Duhem's thesis, then original, has been confirmed through the studies of A. C. Crombie, Adriano Carugo, and William Wallace.

Duhem, when confronted with the evidence, apostasied, changed his mind and provided important proof that the Middle Ages certainly built and informed the era of modern science.