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INTRODUCTION TO JEAN BURIDAN’S LOGIC
7.4 Divided Modal Consequences
In TC 2.2.5 Buridan discusses the equipollence of the various modes
in combination with negations, and in Theorem II-1 he establishes such
equipollences as a formal result.
There are no surprises; ‘necessary’ is
equipollent to ‘not possibly not,’ and so forth.
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Yet there is an impor-
tant methodological corollary of such equipollences which Buridan states in
TC 2.6.7: theorems for divided modals need be stated only for the case in
which the mode is affirmed.
Conversions of divided modals are treated in Theorem II-5 and The-
orem II-6. The results are straightforward: affirmative divided modals de
possibili convert both simply and accidental (to affirmative divided modals
de possibili, of course); universal negative divided modals de necessario con-
vert simply. No other conversions are explored.
Buridan does, however, explore ‘mixing’ theorems, that is, theorems
about which consequences obtain between sentences of different modes. In
Theorem II-3 the relation between assertoric sentences and divided modals
de necessario is stated: the only consequence which obtains is that from
a universal negative. In Theorem II-4 the relation between assertoric sen-
tences and divided modals de possibili is stated the only consequence which
obtains is that from an affirmative assertoric to a particular affirmative de
possibili. Buridan specifically remarks the lack of consequential connection
between divided universals de possibili and their corresponding assertorics
(TC 2.7.32), and between divided particulars de possibili and their corre-
sponding assertorics (TC 2.7.33).
In TC 2.6.33 Buridan defines the mode ‘contingent’: it is equipol-
lent to ‘possibly and possibly-not.’ This mode may also figure in divided or
composite modals, and Buridan explores its behavior in Theorem II-7, The-
orem II-8, Theorem II-14, and Theorem II-19. His motivation for so doing
is not clear; by equipollence, theorems about composite or divided de pos-
sibili sentences will settle questions about contingents as well. The formula
is simple: replace the contingent sentence with a conjunction of de possibili
sentences. Because this theoretical simplication is available, we shall not
discuss Buridan’s treatment of contingent sentences, either as consequences
or as syllogistic.
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The equipollence of necessity and possibility requires divided modals de necessario to
ampliate their subject-terms to stand for possibles, as Buridan points out in Theorem
II-2; this in turn supports his remark in TC 2.6.22 that a de possibili divided modal
follows from a de necessario divided modal, but not conversely.
c Peter King, from Jean Buridan’s Logic (Dordrecht: D. Reidel 1985) 3–82.
INTRODUCTION TO JEAN BURIDAN’S LOGIC
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7.5 Composite Modal Consequences
Buridan begins his discussion of composite modal consequences with
Theorem II-9, which states that the following consequence-scheme is accept-
able:
Some [dictum] is [mode]; therefore, every [dictum] is [mode].
Thus “Some [sentence] ‘The sentence written on the wall is false’ is possible;
therefore, every [sentence equiform to] ‘The sentence written on the wall is
false’ is possible” is an acceptable consequence. In particular the sentence
which is the sole inscription on the wall ‘The sentence written on the wall is
false’ is possible. This view does not land us in paradox; recall that to say
a sentence is “possible” is to say that it describes a possible situation. The
particular inscription on the wall does just that: for example, it describes the
possible situation in which only ‘2+2 = 17’ is written on the wall. Sentences
need not describe their own actual situation to be possible. This is the point
of Theorem II-9: if an inscription or utterance is possible, then it describes
a possible situation, and all equiform inscriptions of utterances are equally
possible since they may be taken to refer to that situation.
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The inscription
on the wall “The sentence written on the wall is false” is possible, but never
possibly-true. With this point noted, Buridan’s theorems about composite
modal equipollences and conversions do not pose any special problem.
Conversions of composite modals are straightforward. If the dictum
is the subject, then all composite modals convert simply with the exception
of the universal affirmative composite modal, which is converted accidentally
(Theorem II-10). If the mode is the subject, then all composite modals con-
vert simply with the exception of the particular negative composite modal,
which is not converted (Theorem II-11). Buridan also discusses “conver-
sions with respect to the dictum,” in which the composite modal sentence
itself is not converted, but the dictum of the composite modal is converted.
Such conversions with respect to the dictum are discussed in Theorem II-12,
Theorem II-13, and Theorem Ii-14, and pose no special problems.
As with divided modals, Buridan also offers mixing theorems for
composite modals in relation to assertoric sentences, and in particular, the
dictum of the composite modal. We may simplify Theorem II-15 as follows:
[Theorem II-15 (revised)] (i ) From any composite affirmative modal
de vero there follows its dictum, and conversely; (ii ) from any com-
posite modal de necessario there follows its dictum; (iii ) from any
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Note that this principle requires strong accessibility among possible worlds: only in
S5, in which every possible world is accessible from every other possible world, is a
claim like this acceptable.
c Peter King, from Jean Buridan’s Logic (Dordrecht: D. Reidel 1985) 3–82.