This text develops a philosophical position integrating realism and rationalism. The author meets the principle challenges of realism and exposes the flaws in the criticisms of the antirealists, showing that realists can explain knowledge of abstract objects without supposing we have causal contact with them, that numbers are determinate objects and that the standard counter-examples to the abstract/concrete distinction have no force. Generalizing the account of knowledge used to meet the challenges to realism, the author develops a rationalist and non-naturalist account of philosophical knowledge and argues that it is preferable to contemporary naturalist and empiricist accounts. The book illuminates a wide range of philosophical issues, including the nature of necessity, the distinction between the formal and natural sciences, empiricist holism, the structure of ontology and philosophical skepticism.