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The Problems of Philosophy
The book begins by looking at why most of us believe in the existence of a physical world apart from the sense data we recieved. This is contrasted with the philosophy of idealism, which claims that everything is a mental construct. Russell goes on to look at the validity of induction, and then examines the different types of knowledge - there is knowledge by description and knowledge by acquaintance, knowledge of general principles, a priori knowledge and intuitive knowledge. Plato thought that there were universal Forms, of which the things that we see are merely shadows. Russell discusses the nature of such universal objects, and goes on to look at truth and falsehood and how personal opinion fits in with these. The book concludes with chapters on the limits of philosophical knowledge and the value of philosophy.
It's a small book, with less that 100 pages, which I would say was one of it's main benefits, as it deals with a sustantial part of philosophy without getting bogged down in long arguments. Hence I would say it was ideal as an introduction to philosophy.