The Variational Principles of Mechanics

I'm currently reading The Variational Principles of Mechanics by Cornelius Lanczos, the fourth edition published in 1970. From the preface to the first edition (1949):

The variational principles of mechanics are firmly rooted in the soil of that great century of Liberalism which starts with Descartes and ends with the French Revolution and which has witnessed the lives of Leibniz, Spinoza, Goethe, and Johann Sebastian Bach. It is the only period of cosmic thinking in the entire history of Europe since the time of the Greeks

The Action Principle may be the most profound principle in all of nature. The 18th century origins of the principle were colorful and controversial. Characters who were deeply involved included Émilie du Châtelet, an aristocratic lady; her lover, the writer Voltaire; and Fredrick the Great, King of Prussia! See The Berlin Academy and forgery.
Du Châtelet was a remarkable woman, her French translation of Isaac Newton's great work Principia Mathematica is still considered definitive. Café Gradot was the meeting place for intellectuals in Paris at the time, many of them her friends. When she tried to join her friends at their table, the management threw her out - women were not allowed in the cafés at the time. Undeterred, on later occasions she arrived attired as a man, and was able to participate in the discussions at the cafe without further incident.