Kim Williams invited me to publish a selection of conferences which I had presented at various occasions during my stay at the UCL and elsewhere. She selected some papers which refer to topics on the history of science, especially on the history of mechanics. My occupation with the history of science began through the mathematician Andreas Speiser, my uncle, at the time the General Editor of the Opera Omnia of Leonhard Euler, who invited me to edit volume III, 5 of the Opera, containing Euler’s work on physical optics. I accepted this offer with great pride and indeed, the task gave me much pleasure and satisfaction. Of all the scientists with whom I entered into contact, aside from Andreas Speiser, I must mention especially Alexandre Koyré. I had the great fortune to meet him at the Institute for Advanced Study in Princeton where we were both guests during the same period. Koyré also introduced me into questions dealing with philosophy of science, an important field, though one that plays a minor role in this present volume. Later I was invited more and more often to give lectures concerning a given subject, or one I working on at the moment.