The New Paradigm and Mental Models

In a recent article in this journal, Johnson–Laird and colleagues argue that mental models theory (MMT) can integrate logical and probabilistic reasoning [1]. We argue that Johnson-Laird and colleagues make a radical revision of MMT, but to ill effect. This can best be seen in what they say about truth and validity (Box 1). Formerly ([2], p. 651), in MMT p ∨ q (p or q) ‘... is true provided that at least one of its two disjuncts is true; otherwise, it is false.’ Thus p ∨ q is true provided that one of three possibilities is true: p & not-q, not-p & q, p & q. However, Johnson-Laird et al. claim, ‘The disjunction is true provided that each of these three cases [p & not-q, not-p & q, p & q] is possible.’ However, these three cases are always possible for jointly contingent statements: that is why they are rows of the truth table for p ∨ q. This new definition makes almost every disjunction true. An example of a disjunction that it does not make true is p ∨ not-p. This tautology fails to be true for their account because p & not-p is not possible.