The Annotated Turing: A review of reading

Charles Petzold’s An Annotated Turing was a book I had been looking forward to reading for a while. I felt I basically knew the central tenants of Turing’s Universal Machine and the halting problem (a phrase not used by Turing), but I lacked an understanding how the ideas were built up and how they were implemented.

This happens a lot with computer science. Basic algorithmic processes are simple to explain and the general operations of computer might be outline-able by the intermediately skilled user, but where the rubber meets the road is glossed over. This gap from generalization to specifics is most evident in blogs / comment threads written by technical interviewers who find that their Magna Cum Laude CS graduate interviewee can’t program FizzBuzz.

For my part, I had watched a visibly pleasing YouTube clip on Turing’s halting problem, I knew the requirements of a Turing machine, and had even read through parts of Introduction to Functional Programming through Lambda Calculus, so I felt I would be pretty comfortable finally settling down with Turing’s paper “On Computable Numbers, With an Application to the Entscheidungsproblem” guided by Petzold.

As Petzold explains at the beginning of the book, Turing’s paper could be reassembled from Petzold’s annotated version. Petzold provides a fairly thorough, but hurried explanation of the math you’ll at least need to have heard of to continue with certain sections of the paper to build up to chapter 14 – “The Major Proof.”

And this is where I fall off, and my biggest take-away from the book occurs, albeit independent of the its subject matter.

In chapter 14, as Turing comes to the conclusion that the Entscheidungsproblem is impossible, I felt nothing. Throughout the book, I knew I was missing some concepts and that I could have spent more time with the unwieldy first-order logic equations that were presented, but that wasn’t the reason I didn’t respond with “Ah! Of course!” when Turing reached his conclusion.

Instead, it was because the entire time, I was focused on how the book could be building to the YouTube videos. And for a variety of reasons, it just wasn’t there. I kept looking and assuming that certain parts were clues to what I knew rather than simply listening to what Turing was saying in the moment.

Above, I said that there is a huge difference between general understanding and detailed understanding. While there is nothing wrong with former as it eventually leads to the details, but it was an error on my part to assume that general understanding was understanding, and I distracted myself by demanding that the specific meet the general somewhere.

It’s easy to hold onto the general understanding as something solid, but to move between different levels of detail requires some degree of abandonment.

It’s the difference between “knowing about” and “knowing” a topic, and Annotated helped me understand not so much that that difference existed, but that failing to incorporate that understanding in how you read or digest a new topic can block the shift from one place to another.

Nonetheless, despite my troubles, Annotated is a worthwhile read, even for a not so worthy reader.