The foreword (here in pre-print version) introduces the scope of this special issue, that is, a re-interpretation of the development of 18th-century German philosophy. We aim in particular at identifying naturalistic and ‘scientific’ tendencies, which evolved alongside the well-studied mainstream currents. In our view, this long-overshadowed manifold phenomenon had two major traits. First, it involved a more intense dialogue between philosophical thought, mathematics, science and medicine. Second, it was pervaded, despite the different positions of its major and minor protagonists, by a shared fundamental naturalistic position in metaphysics and epistemology.