The Annotated Turing

About the book

The Annotated Turing takes a single document as its subject: Alan Turing's 1936 paper on computable numbers, the work that introduced what came to be called the Turing machine. Charles Petzold, known for writing about programming and computing for general readers, reproduces the paper and surrounds it with explanation. The annotations unpack the mathematical language, the notation and the logical steps that a modern reader is likely to find forbidding on a first pass.

Petzold stays with the text. He explains the problem Turing set out to solve, the idea of a machine defined by a finite set of rules, and the reasoning that connects computation to questions about what can be decided by formal methods. The commentary supplies context and links the paper's ideas to later work in computer science, though the center of gravity remains Turing's own exposition.

Because the paper is the spine of the book, the reading experience is closer to a guided seminar than to a popular history. Definitions are examined slowly, notation is explained, and arguments are reconstructed step by step.

How to read it

Approach the book with patience and a pencil. The annotations are designed to be read alongside the paper, so moving back and forth between Turing's text and Petzold's explanations is part of the method. A reader who skims the commentary and jumps to conclusions will miss the care taken with definitions and the sequence of the argument.

The difficulty is uneven. Some passages are accessible to an interested general reader; others demand comfort with formal notation and mathematical reasoning. Petzold does not assume a specialist background, but he does not flatten the material either. Where the argument turns technical, slow reading pays off, and rereading a paragraph often works better than pushing through it.

Who it suits

This book suits readers who have encountered the Turing machine as a name or a diagram and want to know what the original argument actually says. Programmers, students of computer science and readers with an interest in the foundations of mathematics will find the line-by-line treatment useful. It also fits readers who prefer primary texts with expert guidance over secondhand summaries. Those looking for a broad history of computing or a biography of Turing should look elsewhere.

Reading considerations

The book is demanding in places, and its pace is set by the paper rather than by narrative momentum. Readers without some tolerance for formal notation may find sections heavy going. Editions and printings vary in pagination and supplementary material, so check which version you have if you plan to follow along with a separate copy of the paper. The annotations interpret Turing's argument, and readers should test those interpretations against the text itself.