This chapter argues that the most promising way to integrate the history and philosophy of science is the historicist-hermeneutic approach. It presents the main features of the historicist-hermeneutic approach, and shows how it can provide a mutually beneficial integration of the History of Science and the Philosophy of Science. The chapter discusses the historicist-hermeneutic approach to elucidate one of the most problematic historical case studies in philosophy of science: namely, Jean Baptiste Perrin’s argument for molecular reality, which he formulated at the beginning of the twentieth century. Perrin argued that the best way to connect the observable characteristics of Brownian movement with the kinetic-molecular movements causing them was to consider the suspended particles as giant molecules. Perrin attempted the experimental determination of the osmotic pressure of a Brownian particle, in early 1908 and published his results in the Comptes Rendus de l’Académie des Sciences.