The first part is somewhat dry, Russell clearly prefers using precise language when describing numbers, cardinality, and relational sets over analogies and other less precise, but perhaps more accessible, language. I easily forgive him for that, after all he was one of the founders of analytic philosophy, but what knocked the book down in my eyes, surprisingly enough, were his logical flaws at the end in his intro to logic!
"Now if Fx is sometimes true, we may say there are x's for which it is true, or we may say 'arguments satisfying Fx exist'. This is the fundamental meaning of the word 'existence'."
Yeah, he defined existence as something that exists when it is an argument satisfying a function. Things exist because they exist. And I can't see anywhere on the interwebs anyone calling him on this. Or sure, there's the dance around 'functions' and whatnot, but the end result is the definition contains the word it is defining.
He then goes on to conflate material existence with all forms of existence, and declares that there is nothing else aside from the material existence as seen through our senses, "There is only one world, the 'real' world[.]"
Which was exceptional odd since he just spent a chapter talking about the unknown reality of the axiom of infinity.
Russell goes on talking about how while "unicorn" has some meaning, but "a unicorn" does not, it describes nothing. And as far as I can tell, he means nothing material. Again, this confuses me since I have never seen a material existence of a cardinal number aleph. "To say that unicorns have an existence in heraldry, or in literature, or in imagination, is a most pitiful and paltry evasion." To say cardinal numbers have an existence in math books, or lectures, or in imagination, is a most pitiful and paltry evasion. The very arguments he marshals against non "real" objects can be applied to anything non material, including mathematical axioms and concepts.
Russell also lumps "unicorn" with "a round square". While the latter is violation of Euclid's axioms, the former isn't. Instead, a unicorn is something that might have existed in the past and it might exist in the future, but for the present we have no current material evidence. Conflating the two might have been due to limited scope of the book but since he had no qualms challenging Leibniz throughout the book it seems he'd could have easily clarified that point. He either didn't but gave no explanation or he failed to grasp the error; am I the only one that was amused by the irony of this category error?
In other words, while makes sense to dismiss the concept of a round square based on the clear violation of axioms, one cannot do the same with a unicorn. They are clearly in different categories, applying the same reasoning is a logical error. A unicorn wouldn't violate any Euclidean axioms, so it cannot be lumped with something that is in violation.
Despite this ideological flaw that colors his philosophy, it's an interesting and thought provoking work, well worth read for anyone that likes math, is interested in class theory, and enjoys reading what some consider to be a summary of Principia Mathematica.