A philosophy is an integrated whole; it is a system of ideas, every one of which depends on all the others. To prove a philosophy, however, one must break this whole up into parts that are not thus interdependent; one must discover ideas that can stand before one knows the total, ideas that depend only on certain logical antecedents going back ultimately to sense–perception. This lecture, originally given at the Jefferson School Conference in 1987, explains how to reconcile these two features of philosophy, and concludes with a statement of the logical structure of the principles of Objectivism.