Originally published in 1931, this book forms the first of two volumes based on the Gifford Lectures delivered at the University of Edinburgh in 1919 and 1921. The second volume, God and Nature, was originally published in 1952. The text provides a philosophical discussion of the nature of experience, examining the fundamental principles of knowledge regarding the physical world, the self and minds other than our own. Throughout this discussion, a carefully defined 'common sense' position is put forward as the mediating factor in the relationship between the mind and the material world. This book will be of value to anyone with an interest in philosophy, psychology and theories of knowledge.