This introduction presents an overview of the key concepts discussed in the subsequent chapters of the book. This book focuses on the question of what constitutes the character of experience. The main views in the philosophy of perception are defined by how they answer this question. The book covers four of them: the sense datum view, the internal physical state view, the representational view, and naive realism. It talks about the debate between contemporary naive realism and representationalism. Traditionally, naive realism had been cast aside because of internally-generated illusions and hallucinations. But contemporary forms of naive realism show promise in accommodating illusions and hallucinations. Many experiences are essentially “externally directed”. But experiences are also “internally dependent”. Experiences spring from the inside but they also point outward. And this is puzzling. The puzzle comes in two forms. One concerns abnormal perception: illusion and hallucination. The other concerns the experience of “sensible properties” in normal perception.