This book presents an original comprehensive approach to some of the most difficult problems concerning sense-perception and other mental states. After rejecting prevailing approaches, the author presents his own viewpoint which may be characterized as direct, critical realism. Basing his conclusions on conceptual analysis, psychological evidence and historical considerations, the author is able to offer new insights into traditionally unsolved problems concerning the nature of perceptual states, the ontological status of perceptual environment, the cognitive mechanism in perception and the explanation of perceptual mistakes. The book also discusses the implications of this approach for other mental states such as memory and consciousness.