The present chapter focuses on Vilfredo Pareto’s writings during the decades 1870 and 1880, i.e. the period between his early writings on political economy, which were informed by readings of Comte, Mill, Spencer and Cairnes, and his better-known writings on mathematical economics. An examination of this rather unknown period of his intellectual activity shows that Pareto already had at his disposal the elements that he would develop in his later works: the logico-experimental method, the progress of science by successive approximations, the liberal vision and the idea that economic phenomena are only a part of social phenomena.