The fundamental principles of Locke's philosophy are presented in An Essay Concerning Human Understanding (1690), the culmination of twenty years of reflection on the origins of human knowledge. According to Locke, what we know is always properly understood as the relation between ideas, and he devoted much of the Essay to an extended argument that all of our ideas—simple or complex—are ultimately derived from experience. The consequence of this empiricist approach is that the knowledge of which we are capable is severely limited in its scope and certainty. Our knowledge of material substances, for example, depends heavily on the secondary qualities by reference to which we name them, while their real inner natures derive from the primary qualities of their insensible parts.
Nevertheless, Locke held that we have no grounds for complaint about the limitations of our knowledge, since a proper application of our cognitive capacities is enough to guide our action in the practical conduct of life. The Essay brought great fame, and Locke spent much of the rest of his life responding to admirers and critics by making revisions in later editions of the book, including detailed accounts of human volition and moral freedom, the personal identity on which our responsibility as moral agents depends, and the dangers of religious enthusiasm. One additional section that was never included in the Essay itself is Of the Conduct of the Understanding, a practical guide to the achievement of useful beliefs about the world.
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) Jack's undergoes an important transformation from idealist to realist.
