"Is there any knowledge in the world which is so certain that no reasonable man could doubt it?" (Chapter 1, Appearance and Reality, 7)

"All knowledge, we find, must be built up upon our instinctive beliefs, and if these are rejected, nothing is left." (Chapter 2, The Existence of Matter, 25)

"The most natural, though not ultimately the most defensible, hypothesis to adopt in the first instance, at any rate as regards visual sense-data, would be that, though physical objects cannot, for the reasons we have been considering, be exactly like sense-data, yet they may be more or less like." (Chapter 3, The Nature of Matter, 34)

"This question of the distinction between act and object in our apprehending of things is vitally important, since our whole power of acquiring knowledge is bound up with it." (Chapter 4, Idealism, 42)

"Every proposition which we can understand must be composed wholly of constituents with which we are...