Rhetoric and poetics have controlled imagination and phantasm from antiquity until the end of the 18th century. The fantastic has repeatedly broken out to shape the semantic fields of the exorbitant, excessive, impossible, and inconceivable that encompass humanity and its culture. Following the conceptual history of the rhetorical restraint of the fantastic, the analysis of literary texts from the pre-Romantic period to neo-fantastic literature shows how secret knowledge (alchemy, magnetism) and (uncanny) knowledge (medicine, psychology, physics) stimulate phantasms of the supernatural and hidden, leading to speculations with (para-)logical parameters into unprecedented, alternative worlds.