After Gassendi and Descartes, thinkers who tried to explain and study the association of ideas first saw in this phenomenon a cause of error and madness. Progressively, though, whether it was described as "association" or as "connection" of ideas, it began to be seen as one principle of knowledge among others, until Condillac and Hume thought of it as the only principle of all our knowledge, and until xixth century psychology made it its fundamental law. But prior to the full appropriation of this principle by xixth century psychology, early modern thinkers and philosophers have argued and hesitated about the nature of this phenomenon and its correct explanation. The papers edited here intend to study different kinds of problems that arise when one tries to define and give an account of this connection, as well of its various effects. Indeed, the connection of ideas disturbs some well-entrenched oppositions – between subject and object, nature and custom, or mechanical and spiritual – and urges us to reconsider the faculties of the mind.