20
INTRODUCTION TO JEAN BURIDAN’S LOGIC
under which we are in toiuch with an object.
Intentional verbs behave in an odd way: when a term follows the
verb
32
we have an opaque context: Coriscus may not know the one ap-
proaching is his father and he knows his father; the syllogism is prevented
by the apellation of a ratio, and we can conclude only that Coriscus does not
know his father as the concept ‘the-one-approaching’ applies to him, which
is perfectly acceptable. Buridan describes this appellation as similar to ma-
terial supposition (TS 3.8.30, TC 3.7.6), for substitutivity is prevented. On
the other hand, when the term precedes the verb, it is said to appellate
all its rationes indifferently (Rule App-5 in TS 5.3.1 and the discussion;
TC 3.7.7): in this case substuitutivity is preserved and we have a transpar-
ent reading:
33
the sentence “The one approaching is someone Coriscus does
not know” is false, for the ratio ‘the-father-of-Coriscus’ equally applies to
the one approaching.
Buridan’s analysis permits the inference a parte priori to a parte post
for some ratio, which we shall call the Entailment Principle. The converse
entailment a parte post to a parte priori, generally fails, as the nature of
opacity suggests. but in certain cases the latter inference does hold, and
in particular for the verb ‘know’ (scire); we shall call this the Converse-
Entailment Principle (Soph. 4 Remark 8, TS 3.8.27). The objectual version
allows us to infer from “Socrates knows A” the sentence “There is an A
Socrates knows,” and the sentential version (Sophism 13) allows us to infer
from “Socrates knows A to be ϕ” the sentence “There is an A Socrates knows
to be ϕ.” Buridan’s key argument for the Converse-Entailment Principle is
that we should otherwise have to deny that we have knowledge of items in
the world.
34
The Converse-Entailment Principle runs into two difficulties: coun-
terintuitive substitution-instances, and the lack of existential import (i. e.
when no A exists the sentence should be false).
Buridan takes up the first difficulty in Soph. 4 Sophism 14: Socrates,
32
This is Buridan’s grammatical way of drawing scope distinctions: a term appears a
parte post and so in the scope of the verb, or a parte priori and so outside the scope
of the verb.
33
Strictly speaking it is incorrect to call the ‘opaque’ and ‘transparent,’ for they are
not alternative ways of reading one and the same sentence but rather Buridan’s way
of regimenting the difference between the logical form of two different sentences; I
shall use these terms as convenient abbreviates for distinguishing the two classes of
sentences, which should not occasion any confusion.
34
Note that Buridan is careful to state the Converse-Entailment Principle only for scire:
it clearly fails for most intentional verbs, which allow for intentional inexistence.
c Peter King, from Jean Buridan’s Logic (Dordrecht: D. Reidel 1985) 3–82.
INTRODUCTION TO JEAN BURIDAN’S LOGIC
21
who has been studying astronomy, has been imprisoned and cannot see the
sky.
We are permitted in this case to pass from “Socrates knows that
some stars are above the horizon” (by his astronomical studies) to “There
are some stars Socrates knows to be above the horizon.” Which stars?
Those which are in fact above the horizon, which in the posited case is the
constellation Aries. But surely this seems false, for Socrates cannot see the
sky.
Buridan’s reply is to insist on the different reading a parte priori and
a parte post. The constellation Aries is indeed what Socrates knows, but
he knows it only under the complex ratio ‘some-stars-above-the-horizon,’
according to the Entailment Principle. This ratio will of course latch onto
some actual stars, though Socrates does not know which. The Converse-
Entailment Principle allows us to infer “[There are] some stars [which]
Socrates knows to be above the horizon” and, since sustitutivity works a
parte priori, we may infer from the fact that the constellation Aries above
the horizon “The stars of Aries Socrates knows to be above the horizon.”
But the Entailment Principle licenses us to pass back only to “Socrates
knows stars (under some ratio) to be above the horizon,” and the ratio in
question is ‘some-stars-or-other.’ This, Buridan holds, is not counterintu-
itive at all but the natural view of the matter.
But this answer might seem to be a cheat.
35
For “it surely trades
on the peculiar characteristic [of scire] in that what you know must be so.”
Yet this is exactly what Buridan has been emphasizing all along, and is the
very reason why the Converse-Entailment Principle holds only for scire.
The second difficulty mentioned above was that it seems I can know
that thunder is a sound in the clouds even in the absence of any thun-
der, but by the Converse-Entailment Principle then “Any thunder I know
to be a sound in the clouds” should be true, and in the posited case the
subject-term is empty; affirmative sentences with empty subject-terms are
automatically false (TC 2.3.3, QM 4.14 fol. 23va).
36
Buridan’s answer in-
volves his theories of ampliation and natural supposition, which we shall
discuss below in Section 6.5.
35
Which is exactly what Geach [1972] 134 calls it.
36
by the Square of Opposition, negative sentences with empty subject-terms are auto-
matically true. Hence “The present King of France is bald” is false, and “Pegasus is
not a winged horse” is true.
c Peter King, from Jean Buridan’s Logic (Dordrecht: D. Reidel 1985) 3–82.