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term “human” is the middle term(M) that links together the promises. “animal”
is the subject(S) of the conclusion, and “mortal” is the predicate(P) of the
conclusion. The arrangement of the middle term in each premise yields four
types of scheme for reasoning, known as the figures of syllogism, that is, the
figures M-P/S-M, P-M/S-M, M-P/M-S and P-M/M-S.
There are two research paradigms in studying how humans perform syllogis-
tic reasoning: one is conclusion-evaluation paradigm, which requires subjects to
decide whether a presented conclusion is deduced from the premises and the
other is conclusion-generation paradigm, which requires subjects to generate the
conclusion following the premises. Researchers have found that the evaluation
task is more prone to conclusion-drive processing and the generation task may
be viewed as a premise-driven processing [14].
Frase [9] performed earlier research about figures of syllogism with the
conclusion-evaluation paradigm. The results show that the figure M-P/S-M/S-P
takes significantly less time and produces fewer errors than the figure P-M/M-S/S-
P. Mediated association theory [9] is proposed to explain the results. As showed
in Fig. 1, the figure M-P/S-M/S-P is a forward-chaining paradigm with the re-
sponse chaining of S-M-P, while the figure P-M/M-S/S-P is a backward-chaining
paradigm with response chaining of P-M-S when inferring the conclusion with the
direction of S-P. Late studies on syllogism have demonstrated that the figure of
premises has strong influence on syllogistic reasoning behaviors [2,3,12].
Johnson-Laird and Bara [12] reported a study on syllogism with the conclusion-
generation paradigm. They stated that figural effects occur when subjects inte-
grate premises. The mental model theory (MMT) [13] suggests that the figure
P-M/M-S where the middle terms are contiguous is less cognitively demanding
than the figure M-P/S-M where the middle terms are not. Furthermore, the fig-
ure M-P/S-M has a response bias of the conclusion with direction of S-P, while
the figure P-M/M-S has a response bias of P-S. Unlike MMT, the dual mechanism
theory [5,8,10,19], proposes that there are two distinct cognitive systems underly-
ing reasoning. System 1, which is shared with other animals, comprises a set of au-
tonomous subsystems that include both innate input modules and domain-specific
knowledge acquired by a domain-general learning mechanism, and is considered as
associative processing. System 2, which is specific to humans and constrained by
working memory capacity, is considered as reflective processing and participates
Fig. 1.
Response chain of figure M-P/S-M/S-P and figure P-M/M-S/S-P