Synthesized answer
The central thesis of "How to Solve It" by George Pólya is that the mathematical method of demonstrating a proof or finding an unknown can be applied to attack any problem that can be reasoned out [1]. The book aims to show anyone, regardless of their field, how to think straight [1, 2].
Pólya's instructions focus on stripping away irrelevancies and going directly to the core of a problem [1, 2]. The book also includes a heuristic dictionary with advice on making problems more manageable [2]. While the passages explain the broad applicability and methodology of the book, they do not explicitly state a single, overarching thesis sentence.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: How to solve it by George Pólya Description: A perennial bestseller by eminent mathematician G. Polya, How to Solve It will show anyone in any field how to think straight. In lucid and appealing prose, Polya reveals how the mathematical method of demonstrating a proof or finding an unknown can be of help in attacking any problem that can be “reasoned” out—from building a bridge to winning a game of anagrams. Generations of readers have relished Polya’s deft—indeed, brilliant—instructions on stripping away irrelevancies and going straight to the heart of the problem. --- Google Books…
How to Solve It includes a heuristic dictionary with dozens of entries on how to make problems more manageable—from analogy and induction to the heuristic method of starting with a goal and working backward to something you already know. This disarmingly elementary book explains how to harness curiosity in the classroom, bring the inventive faculties of students into play, and experience the triumph of discovery. But it’s not just for the classroom. Generations of readers from all walks of life have relished Polya’s brilliantly deft instructions on stripping away irrelevancies and going…