Handbook of the History of Logic: - Fordham University Faculty

332 Henrik Lagerlund
the conclusion. A dialectical syllogism, however, is one that produces
opinion on the basis of probable [premises]. Finally, a sophistical syllogism
is one that either syllogizes on the basis of seemingly probable
[premises] or seemingly syllogizes on the basis of probable [premises];
in either case it is strictly aimed at glory and victory. 91
This division is very similar to Al-Ghazālī’s discussed above. William never deals
with demonstrative syllogisms, however, but, as Peter, turns to the dialectical ones
after having outlined the theory of the syllogisms.
The treatment of the loci (or topoi) in thirteenth century textbooks deals with
the same issues as Aristotle dealt with in the Topics, but the medieval discussions
of the Topics had changed quite a lot, however, and it was Boethius that set the
subject matter of this part of logic for the Middle Ages. The textbooks author’s
discussion of loci is hence based on Boethius’ discussion in De differentiis topicis.
Peter of Spain defines an argument in the following way:
An argument is the ratio that makes a doubtable thing credible.
The ratio, or the reason’, is the medium for deriving or reaching the conclusion of
the argument. The notion of a medium or a middle is brought out most explicitly
in the theory of the syllogisms where the middle term is the link between the
extreme terms. The conclusion of an argument is an approved proposition and
before it was proved it was doubtable. This way of describing arguments from
premise to conclusion is explicit in Aristotle’s Topics where the starting point is
a question that is tested by the argument.
An argumentation is an argument made explicit by a discourse, that is, the
discourse that explicates the argument. An argument differs from both a medium
and an argumentation, Peter explains. A medium is called a medium because it
has two extremes, while an argument adds to the medium the power of proving
the conclusion. In order for there to be an argument there must be a medium
able to prove the conclusion. The argumentation, however, is the whole discourse
composed of premises and conclusion, and in an argumentation the power of an
argument is manifested. The argumentation is thus the whole uttered discourse
of premises and conclusion. It is an argument, which is proved by a medium from
the two extremes in the premises.
There are four kinds of arguments, according to Peter, namely syllogisms, induction,
enthymems, and examples. He does not discuss syllogisms, but gives
short examples of the other three. An induction is an argument from a particular
proposition to a universal. An enthymem is an imperfect syllogism, like:
91 William of Sherwood, Introduction to Logic, 69. This division is also explicit in Roger
Bacon’s textbook. He also has a long discussion of demonstrative syllogisms, which is very
interesting and seems influenced by Al-Ghazālī. He also discusses something he calls syllogismus
falsigraphicus and also dialectical and sophistical syllogisms. All these are, according to him,
divisions according to the matter of the syllogism. See Roger Bacon, Summulae Dialectices, III,
207-20.