Stathis Psillos (Athens) gives a talk at the MCMP Colloquium (13 December, 2012) titled "Making Contact with Molecules: On Perrin's Argument for Realism". Abstract: Between roughly 1908 and 1912, there was a turn in the scientific community in favour of the atomic hypothesis. Jean Perrin’s theoretical and experimental work on the causes of Brownian motion played a major role in this shift. When Perrin received the Nobel Prize for physics in 1926, it was noted in the presentation speech that he “put a definite end to the long struggle regarding the real existence of molecules”. The aim of this talk is to cast light on the reasons that explain the shift of opinion concerning the reality of atoms and molecules in the beginning of the twentieth century. The story told will have some rather interesting repercussions concerning the scientific realism debate. After presenting the philosophical debate concerning the role and status of explanatory hypotheses c.1900, (focusing on the work of Ostwald, Poincaré and Boltzmann), I will examine in detail Perrin’s theoretical account of the molecular origins of Brownian motion and explain the structure and the strength of Perrin’s argument for the reality of molecules.