Throughout history, people have tried to construct 'theories of everything': highly ambitious attempts to understand nature in its totality. This account presents these theories in their historical contexts, from little known hypotheses from the past to modern developments such as the theory of superstrings, the anthropic principle and ideas of many universes, and uses them to problematize the limits of scientific knowledge. Do claims to theories of everything belong to science at all? Which are the epistemic standards on which an alleged scientific theory of the universe - or the multiverse - is to be judged? Such questions are currently being discussed by physicists and cosmologists, but rarely within a historical perspective. This book argues that these questions have a history and that knowledge of the historical development of 'higher speculations' may inform and qualify the current debate of the nature and limits of scientific explanation.