The rise of modern science during the seventeenth century shook traditional beliefs in religion, politics and morality, at the same time instilling into those who renounced those beliefs an unforeseen conviction of the power and scope of the human intellect. But science brought with it a new and unfamiliar bridle to the ambitions of thought. It rested its authority at least in part on observation. This gave new impetus to the Cartesian doubt. If what I know of the world I know through observation, then what can I know beyond the fact that I seem to observe things? In other words, what can I know beyond the contents of my own mind? Without the overarching structure of a priori truth, philosophy seems to lack the bridge that will take it from subject to object. It lies trapped in the first person, forced either to remain there, or to call, like Berkeley, in some new and less reasonable way, upon the God who had rescued Descartes from solipsism.