(Copyright © 2000 Piero Scaruffi | Legal restrictions - Termini d'uso )
Johnson-Laird outlines a number of psychological experiments which prove that the mind does not reduce all mental operations to mathematical logic. Instead propositional reasoning seems to rest on what he calls mental models.
Precisely, Johnson-Laird's representational theory assumes that mind represents and processes models of the world. The mind solves problems without any need to use logical reasoning. A linguistic representation such as Fodor's is not necessary.
A sentence is a procedure to build, modify, extend a mental model. The mental model created by a discourse exhibits a structure that corresponds directly to the structure of the world described by the discourse.
To perform an inference on a problem the mind needs to build the situation described by its premises. Such mental model simplifies reality and allows the mind to find an "adequate" solution.
Johnson-Laird draws on several phenomena to prove the psychological inadequacy of a mental logic. People often make mistakes with deductive inference because it is not a natural way of thinking. The natural way is to construct mental models of the premises: a model of discourse has a structure that corresponds directly to the structure of the state of affairs that the discourse describes. How can children acquire inferential capabilities before they have any inferential capabilities? Children solve problems by building mental models that are more and more complex.
Johnson-Laird admits three types of representation: "propositions" (which represent the world through sequences of symbols), "mental models" (which are structurally analogous to the world) and "images" (which are perceptive correlates of models).
Images are ways to approach models. They represent the perceivable features of the corresponding objects in the real world.
Models, images and propositions are functionally and structurally different.
Linguistic expressions are first transformed into propositional representations. The semantics of the mental language then creates correspondences between propositional representations and mental models, i.e. propositional representations are interpreted in mental models.
Turning to meaning and model-theoretic semantics, Johnson-Laird proposes that a mental model is a single representative sample from the set of models satisfying the assertion. Semantic properties of expressions are emergent properties of the truth conditions. Johnson-Laird's procedural semantics assumes that there are procedures that construct models on the basis of the meaning of expressions.
Johnson-Laird believes that consciousness is computable. The mind contains a high-level operating system and a hierarchy of parallel processors. Conscious mind is due to a serial process of symbolic manipulation that occurs at the higher level of the hierarchy of processors (in the operating system), while unconscious mind is due to a parallel process of distributed symbolic representation. Emotions are non-symbolic signals, caused by cognitive interpretations of the situation, that propagate within the hierarchy.