Bertrand Russell opens Introduction to Mathematical Philosophy by distinguishing the book's purpose: it is an introduction, not an exhaustive treatise, aimed at presenting results of mathematical logic in a form accessible to beginners. He explicitly avoids dogmatism on unsettled questions, a stance that shapes the selection of topics. The preface notes that matters once philosophical—like infinity and continuity—have become mathematical, and that mathematical logic invalidates much traditional philosophy. This framing sets the tone for a work that is both expository and argumentative, grounding philosophical discussion in technical results.

Definitions and the Avoidance of Dogmatism

Russell's preface establishes a careful rhetorical posture. He writes that the book 'does not aim at giving an exhaustive discussion' and that 'the utmost endeavour has been made to avoid dogmatism on such questions as are still open to serious doubt.' This is not mere modesty; it reflects a methodological principle. The choice of topics is 'dominated' by this endeavour, meaning that areas of settled science are prioritized over speculative frontiers. The result is a text that presents definite results—such as the nature of continuity—while acknowledging where certainty ends. Readers should note that Russell treats mathematical logic as a body of knowledge that 'appears to invalidate much traditional philosophy,' yet he refrains from claiming finality. The preface thus functions as a guide to the book's epistemic limits.

The Language of Continuity and Limits

In the excerpt on continuity, Russell employs a precise, definition-driven style. He defines a function as continuous at an argument if, for every positive number σ, there exists a positive number η such that for all values of δ numerically less than η, the difference f(x+δ) – f(x) is numerically less than σ. The language is iterative and conditional, building definitions step by step. He contrasts this with the 'exceptionally tame' function that has a definite limit, noting that the general rule is oscillation. The passage illustrates Russell's method: he starts with a concrete definition, then examines its implications, often by considering what happens 'as the argument approaches some value from below.' This approach mirrors the book's overall structure—moving from simple definitions to complex consequences, always with an eye to philosophical import.

Oscillation and the Ultimate Section

Russell extends the discussion of continuity by introducing the concepts of 'ultimate section' and 'ultimate oscillation.' He considers a function as the argument approaches a value from below, defining the set of values for arguments in an interval (a–ε, a). The 'ultimate section' is the common part of all sections for all possible ε. To belong to this section means that, however small ε is, there are arguments in (a–ε, a) for which the function value is not less than that number. A parallel 'ultimate upper section' is defined for values not greater than the function. If a number belongs to both, it is part of the 'ultimate oscillation.' This construction is a technical tool for analyzing functions without assuming limits. Russell's prose remains precise but avoids excessive formalism, using phrases like 'we may illustrate the matter by considering once more the function sin(1/x) as x approaches 0' to ground the abstraction. The passage exemplifies how the book balances rigor with accessibility.

Readers should approach this book as a primer that demands careful attention to definitions. Russell's prose is dense but not opaque; each term is introduced with explicit conditions. The excerpts show a pattern: a definition, a contrast with simpler cases, and then an exploration of the general case. This structure rewards slow reading. The book is not a survey of mathematical philosophy but a focused introduction to its logical foundations, and its value lies in the clarity of its distinctions.