Handbook of the History of Logic: - Fordham University Faculty

634 E. Jennifer Ashworth
ing to Jardine and others, is a part of logic primarily concerned with persuasive
techniques and informal argumentation, that is, with non-deductive strategies. In
fact, the link between dialectic and disputation goes back to Aristotle, and was
generally accepted. Domingo de Soto, for instance, discusses the Greek derivation
of the word ‘dialectica’, saying that it comes from ‘disputo vel dissero’ andso
dialectic is a ‘disputatoria scientia’. 125 He applies this assessment to logic as a
whole, rather than to a single part. The important question is whether it follows
from dialectic’s link with the Topics and with disputation that it is concerned with
informal argumentation, and if it is, how far this concern extends.
Before this question can be answered, we need to consider the notion of probability.
The term ‘probabilis’ itself had a fairly general meaning. A probable
proposition was likely, plausible, susceptible of proof though not usually of demonstration
in the strict Aristotelian sense. It was recognized that there are degrees
of belief, and that a probable belief or opinio is one that falls somewhat short of
certainty, while being more robust than mere suspicio, the special province of the
rhetorician. It was also recognized that even those propositions which ought to
be certain might be matters of opinion for some. As Paul of Pergula remarked,
not everyone believes that God exists, even though “God exists” was a standard
example of a necessarily-true proposition. 126 No logician that I know of displayed
any awareness of statistical or relative frequency interpretations, and the notion
of subjective probabilities, according to which premisses support a certain degree
of probability attached to the conclusion, comes up only occasionally, particularly
in the work of Philip Melanchthon, as we shall see in section 4 below. More usually,
however, if an argument was labelled as ‘probable’ this had to do, not with
the replacement of a strict logical relation between premisses and conclusion by
some other relation, but with the epistemological status of the premisses. If at
least one premiss was about contingent matters, or was wrongly taken to be about
contingent matters, the argument, albeit deductively valid, fell into the category
of probable or dialectical syllogism. This reading is, of course, based on the belief
that Aristotle’s reference to ‘syllogism’ in the Topics is to be taken as a reference
to a particular kind of formally-valid deductive structure. Thus, demonstrative
reasoning involves formal syllogisms with certainly-true premisses, and dialectical
reasoning involves the same formal syllogisms with probably-true premisses. Sophistical
reasoning may or may not involve formal syllogisms, since different kinds
of logical error are possible. On this interpretation, dialectic is clearly a sub-part
of logic, or, for those who preferred the term ‘dialectic’ to the term ‘logic’, dialectic
in the narrow sense is a part of dialectic in the broad sense.
The notion that there are three different kinds of syllogism, differentiated not
with respect to their form but with respect to the epistemic status of their premisses,
fitted in with the standard ordering of the Organon. Terms were discussed
125 Soto, Summulae, f. 3 va. Cf. Franciscus Toletus, Introductio in dialecticam Aristotelis in
Opera omnia philosophica I–III (Cologne 1615–1616; repr. Hildesheim: Georg Olms, 1985), p.
4a.
126Paul of Pergula, Logica, p. 88.