Chapter 4 analyzes Tinbergen’s scientific coming of age under the mentorship of the physicist Paul Ehrenfest. Tinbergen started his studies in mathematics and physics in 1921 in Leiden where he encountered some of the leading physicists of the age, such as Heike Kamerlingh Onnes and Ehrenfest. Especially the latter helped turned Leiden into a lively scientific hub with innovations in the curriculum, teaching methods, and his famous inquisitive seminars, which earned Ehrenfest the title the Socrates of Leiden. From 1923 onward Tinbergen was mentored by Ehrenfest, who guided his studies in both physics and economics. The chapter details the difficult choice that Tinbergen had to make between these two fields, as well as the shared vision that Tinbergen and Ehrenfest developed about the role of science and scientists in modern society. In 1929 Tinbergen completed his dissertation in physics, which already contained some analogies between physical and economic phenomena. In the following years Ehrenfest stimulated Tinbergen’s statistical research in economics, which soon became part of the new field of econometrics. The two would host the meetings of the Econometric Society in 1933, but just days before, Ehrenfest would take his own life.