A person doesn't have to look hard to find the main argument from this book, because the title just says it in plain English. This book is Russell's criticism of philosophy, both in an abstract sense and in a concrete discussion of the theories and writings of philosophers past. His core argument is the distinction between reality and what humans are able to learn about reality through their senses and mental faculties.
This distinction is easy to make, but hard to understand. Russell illustrates through this book ways in which he feels that philosophers have failed to take this dilemma into account. For instance, there is the problem of common language games in philosophy. Russell takes an intriguing stance on Platonic idealism, suggesting that he believes that there are potential forms which the human mind "remembers," but he is rather confident that our experience of those potential objective realities is dubious at best.
What that means is basically this: Russell believes that there might be an objective, metaphysical reality, but he doesn't have confidence that humans are approximating it through language games. He feels that humans should hold their philosophies with open hands, because what we know about the universe is extremely limited in every way.
