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How We Reason$
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Philip Johnson-Laird

Print publication date: 2008

Print ISBN-13: 9780199551330

Published to Oxford Scholarship Online: March 2012

DOI: 10.1093/acprof:oso/9780199551330.001.0001

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Unwrapping an Enigma

Unwrapping an Enigma

(p.387) Chapter 26 Unwrapping an Enigma
How We Reason

Philip N. Johnson-Laird

Oxford University Press

This chapter begins by describing the ciphers and the Enigma machine, and the German protocol for setting up the machine for encoding or decoding a message. It was here that they made one fateful error. And then the chapter explores Gordon Welchman's thinking as he broke the code. The Enigma machine had two separate components that created its encodings. Welchman described his thinking in a sequence of steps. These steps are discussed in detail. Each step in Welchman's reasoning can be explained in terms of the major processes of deductive and inductive reasoning that the model theory elucidates. His principal inductions are based on observations of the output of machines. The initial cracking of the Enigma code, which Welchman duplicated, occurred at the same time that other methods for breaking it were under development.

Keywords:   Enigma machine, German protocol, Gordon Welchman, thinking, reasoning, model theory, Enigma code

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