After a Darwinian-type account of what beliefs are and how they arose in animals acting to cope with their environments--"low beliefs," virtually all of which are true--Wallace Matson here shows how the invention of language led to imagination and thence to beliefs formed in other ways ("high beliefs"), not true though thought to be, which could be consolidated into mythologies, the first Grand Unified Theories of Everything. Science began when Thales of Miletus produced a Grand Theory based on low ("everyday") beliefs. Matson traces the course of science and philosophy through seven centuries to their sudden and violent displacement by Christianity with its Grand Theory of the old type. Against the widespread opinion that modern philosophy has slowly but completely emancipated itself from bondage to theology, he shows how remnants from the medieval 'interlude' still lurk unnoticed in the purportedly neutral notions of logical possibility, possible worlds, and laws as commands, to the detriment of the natural harmony between science and philosophy, including ethics. Accessibly written, this is a book for all who are interested in the foundations of 21st century thought and who wonder where the cracks might be.