Throughout history, but most especially during the eighteenth-century Enlightenment, great minds of philosophy grappled with two thorny questions: What are the objects of knowledge? and How do we come to know them? Using the revealing dialogue technique, Berkeley shakes the very ground of those who believe that something called matter exists to support the sensible qualities we perceive. In his critique of this view, Berkeley argues for ideas in the mind as the only true reality about which one can have knowledge. His arguments for these conclusions, and for the ultimate foundation of all sensible things, can be found in this essential work of early modern philosophy.