Synthesized answer
The central thesis of *The Problems of Philosophy* appears to be an intention to present positive and constructive insights on philosophical problems, rather than solely negative criticism [Passage 1]. The author states that the book focuses on problems where it was possible to offer something positive and constructive [Passage 1].
Due to this focus, the theory of knowledge occupies a larger portion of the volume compared to metaphysics [Passage 1]. Consequently, some topics frequently discussed by philosophers are either treated very briefly or omitted entirely [Passage 1]. The provided passages do not explicitly state a single, overarching central thesis in a declarative sentence, but rather describe the author's approach and the book's thematic emphasis.
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.