This is Volume XIX of twenty-one in the Cognitive Psychology series. First published in 1956, this study makes a unique contribution to our knowledge of perception. The author takes a representationalist view and disposes of the theory of projected sensations. Adopting Bertrand Russell’s idea of physical and perceptual worlds, proceeds to discuss the possible geometries of their mutual relations. The value of this book is that the author has made available to students of philosophy much new knowledge of the first importance.