Idealism is one of the oldest philosophies that believes ideas are the true reality, not the material world which is imperfect and changing. The document discusses the key principles of idealism including that spirit and mind constitute reality, values are absolute, and truth cannot be found in the material world. It provides examples of important idealist philosophers from both Western and Indian traditions such as Plato, Descartes, and Aurobindo Ghosh. The metaphysics, axiology, and epistemology of idealism are also summarized.