Someone who has more sympathy with traditional empiricism than with much of present-day philosophy may ask himself: 'How do my experiences give rise to my beliefs about an external world, and to what extent do they justify them?' He wants to refer, among other things, to unremarkable experiences, of a sort which he cannot help believing to be so extremely common that it would be ridiculous to call them common experiences. He mainly has in mind sense-experiences, and he thinks of them in a particular way. His way of thinking of them, roughly speaking as something 'inner', is one on which recent logico-linguistic philosophy has thrown a good deal of light. The relevant special notion of an experience contrasts, among other things, with a certain more general biographical notion of an experience, which some dictionaries indicate by the definition, 'an event of which one is the subject'. This book explores the concept of experiences, focusing on the disjunctions between perception and illusion.