“…Braine & O'Brien, 1998;Rips, 1994), though they differ in the way they treat the relations between the sets. For instance, some theorists make use of diagrammatic representations to handle relations (Ceraso & Provitera, 1971;Erickson, 1974;Ford, 1995;Newell, 1981), others rely on formal rules of inference (Geurts, 2003;Guyote & Sternberg, 1981;Politzer, van der Henst, Luche, & Noveck, 2006;Stenning & Yule, 1997); and yet others analyze sets in terms of simulated possibilities, i.e., mental models (Bucciarelli & Johnson-Laird, 1999;Johnson-Laird & Byrne, 1991;Polk & Newell, 1995). The psychological systems can all account for how individuals make valid deductions, however few of them can account for the differences in relative difficulty between various reasoning problems.…”