This study argues that the 18th century, so long regarded as the age of reason, should also be considered the age of passions. Eighteenth-century writers began to explore self-interest, sociability and love, and to manipulate them in ways that would have momentous consequences for the development of Western culture. When carefully cultivated: self-interest led to prudent behaviour and national improvement; sociability contributed to inter-group harmony and national identity; the powerful attraction between the sexes metamorphosed into politics and altruism.