The principal purpose of this book is to draw a distinction between our judgement about reality and reality itself. To this end, Russell puts forward an epistemological theory of the nature of truth. He first discusses our knowledge of the physical world and how we come to possess it. He claims certain beliefs about a table in his room, and wants to know if these beliefs equate to knowledge. If his beliefs about the table give him actual knowledge of it, what kind of thing, then, is this table? The table is made up of tiny particles of atomic matter and is only a table because his experience of it qualifies it to be so. When he sees the table, he sees a brown shape on "legs". This he calls "sense datum", explaining that it is not the combination of atoms that we are seeing but our sensory interpration of them that makes a table a table. This is empirical knowledge and this is what we use in order to interpret and understand our experience in the world.
This is the opposite of idealism, which is the viewpoint that whatever exists must be at first conceptualized as an ideal before becoming something that we physically experience. Russell puts forward his philosophy of particulars; they are physical objects that are in a particular place at a particular time. Universals are the qualities of these physical objects, for example, size or depth of color. Since idealists believe everything is a mental experience, Russell proposes that they are confusing the act of experience with the object that one is experiencing.
Russell believes that he has knowledge by acquaintanceship and makes a distinction between knowledge gained by being personally acquainted with something, and knowledge gained by having the "something" explained by somebody else. We have actual knowledge of only the things we have experienced and therefore have sensory data for. He then expands on his theory of descriptions. He contends that there are two kinds of terms that we use for an object - its name, and its definite description. His primary example is Bismarck, or "The First Chancellor of Germany". They are the same entity but the description allows us to think about and understand an object, or person, with whom we have no personal acquaintance, and therefore sensory experience, of. This is how we achieve an indirect knowledge of things.
Russell tends to support our inclination to go by "gut instinct" and is a believer in the principle of intuition. He basis this belief on facts, propositions and complexes. Facts are independent of human awareness (best summed up by the familiar question of whether a tree falls in a forest with nobody to hear it still makes a sound or not) Particulars and universals are both propositions. This is a complex philosophical expression of the meaning of things. They can constitute knowledge of something if appropriately considered alongside universals and particulars. This is the most complex part of his philosophy. His theory is that a true proposition is a correspondence between a belief and a fact.
Then, Russell moves on to discuss a priori knowledge. Here, he is somewhat contradictory of his other positions as he agrees with the Platonic ideals that it is possible to have acquaintance with a universal without knowing a thing about it. This makes the principle of priori knowledge accessible and understandable to the beginner or layperson. Basically, we can have knowledge of the general principles of something without having a definite degree of certainty gleaned from personal experience of it. By espousing this Russell is able to intertwine the two seemingly oppositional philosophies of idealism and empiricism.
Russell also gives the reader a general overview of the main principles of philosophy that have gone before. He takes older and generally unresolved philosophical arguments and presents a solution for them using his own analyses. With this he intends for the book to be a springboard for further discussion and debate. He is the only person actually participating in the discussion but manages to act as the moderator between the philosophical voices he is telling the reader about. Thus this is a more accessible book about the key tenets of philosophy than previous works. Each chapter gives us more information and builds on knowledge gained in the chapter before. This then gives us a logical progression of philosophical principles that engender discussion.
