Synthesized answer
The author of "The Problems of Philosophy" focused on problems within philosophy where a positive and constructive approach was possible [1]. Consequently, the book dedicates more attention to the theory of knowledge than to metaphysics [1]. Some topics typically discussed by philosophers are treated very briefly or omitted entirely [1].
The passages do not provide enough information to explain the key concepts in the author's own words. They indicate the book's focus on theory of knowledge and constructive arguments, but do not elaborate on what those key concepts are.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: The Problems of Philosophy by Bertrand Russell Description: In the following pages I have confined myself in the main to those problems of philosophy in regard to which I thought it possible to say something positive and constructive, since merely negative criticism seemed out of place. For this reason, theory of knowledge occupies a larger space than metaphysics in the present volume, and some topics much discussed by philosophers are treated very briefly, if at all.