Originally published in 1974, the second volume of four (Logical Inference: Underlying Operations) provides a process-model for the solution of certain syllogistic reasoning problems. Testable predictions of the model are easily derived, and the available evidence supports the model’s description of the real-time mental steps mediating these logical abilities. A theory of development, connected to the model, makes these volumes all the more important for cognitive, developmental, and educational psychologists, as well as educators and linguists.