The research started by Newell on unified theories of cognition represents a crucial element of divergence with respect to the vision of his long-term collaborator, and AI pioneer, Herbert Simon for what concerns the future of artificial intelligence research.
Antonio Lieto recently drew attention to such a discrepancy,[2] by pointing out that Herbert Simon decided to focus on the construction of single simulative programs (or microtheories/"middle-range" theories) that were considered a sufficient mean to enable the generalisation of “unifying” theories of cognition (i.e. according to Simon the "unification" was assumed to be derivable from a body of qualitative generalizations coming from the study of individual simulative programs).
He also claims the established cognitive models are vastly underdetermined by experimental data.
By cognition, Newell means: After arguing in favor of the development of unified theories of cognition, Newell puts forward a list of constraints to any unified theory, in that a theory should explain how a mind does the following: Newell's secondary task is to put forward the cognitive architecture Soar as an implementation of a UTC that meets the constraints above.
Other efforts at unified theories of cognition cited in the book include ACT-R and the human processor model.