Reminder.
In syllogistics, all terms are nonempty.
Barbari. AaB , B aC : AiC .
Every unicorn is a white horse.
Every white horse is white.
Some unicorn is white.
In particular, this white unicorn exists.
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Axioms of Syllogistics.
Aristotle discusses the first figure in Analytica Priora I.iv,
and identifies Barbara, Celarent, Darii and Ferio as
perfect:
Barbara. AaB , B aC : AaC
Celarent. AeB , B aC : AeC
Darii. AaB , B iC : AiC
Ferio. AeB , B iC : AoC
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Simple and accidental conversion.
Simple (simpliciter ).
X iY
Y iX .
X eY
Y eX .
Accidental (per accidens).
X aY
X eY
X iY .
X oY .
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Syllogistic proofs (1).
We use the letters tij for terms and the letters ki stand for
copulae. We write a mood in the form
t11 k1 t12
t21 k2 t22
t31 k3 t32 ,
for example,
AaB
BaC
AaC
for Barbara. We write Mi for ti1 ki ti2 and define some
operations on moods.
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Syllogistic proofs (2).
For i ∈ {1, 2, 3}, the operation si can only be applied if ki
is either ‘i’ or ‘e’. In that case, si interchanges ti1 and ti2 .
For i ∈ {1, 2}, let pi be the operation that changes ki to
its subaltern (if it has one), while p3 is the operation that
changes k3 to its superaltern (if it has one).
Let m be the operation that exchanges M1 and M2 .
For i ∈ {1, 2}, let ci be the operation that first changes ki
and k3 to their contradictories and then exchanges Mi
and M3 .
Let perπ be the permutation π of the letters A, B, and C,
applied to the mood.
BaA
AaC
BaC
per
))
AaB
BaC
AaC
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Syllogistic proofs (3).
Given any set B of “basic moods”, a B-proof of a mood
M = M1 , M2 :M3 is a sequence ho1 , ..., on i of operations such
that
Only o1 can be of the form c1 or c2 (but doesn’t have to
be).
The sequence of operations, if applied to M , yields an
element of B.
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Syllogistic proofs (4).
hs1 , m, s3 , perAC i is a proof of Disamis (from Darii) :
AiB
CaB
AiC
s1
//
BiA YYeYeYeYmeYeYeYeY22 CaB
e
,,
BiA
CaB
AiC
AiC
s3
CaB
BiA
//
CiA
per
))
AaB
BiC
AiC
hs2 i is a proof of Datisi (from Darii) :
AaB
CiB
AiC
s2
AaB
// BiC
AiC
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Syllogistic proofs (5).
hc1 , perBC i is a proof of Bocardo by contradiction (from
Barbara) :
AoB
CaB
AoC
SSSSc1 kk55
kSkSkSSS
k
k
k
))
AaC
CaB
AaB
per
))
AaB
BaC
AaC
Definition. Let B be a set of moods and M be a mood. We
write B ⊢ M if there is B-proof of M .
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Mnemonics (1).
Bárbara, Célarént, Darií, Ferióque prióris,
Césare, Cámestrés, Festíno, Baróco secúndae.
Tértia Dáraptí, Disámis, Datísi, Felápton,
Bocárdo, Feríson habét. Quárta ínsuper áddit
Brámantíp, Camenés, Dimáris, Fesápo, Fresíson.
“These words are more full of meaning than any that were ever made.” (Augustus de Morgan)
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Mnemonics (2).
The first letter indicates to which one of the four perfect moods the mood is to be
reduced: ‘B’ to Barbara, ‘C’ to Celarent, ‘D’ to Darii, and ‘F’ to Ferio.
The letter ‘s’ after the ith vowel indicates that the corresponding proposition has to be
simply converted, i.e., a use of si .
The letter ‘p’ after the ith vowel indicates that the corresponding proposition has to be
accidentally converted (“per accidens”), i.e., a use of pi .
The letter ‘c’ after the first or second vowel indicates that the mood has to be proved
indirectly by proving the contradictory of the corresponding premiss, i.e., a use of ci .
The letter ‘m’ indicates that the premises have to be interchanged (“moved”), i.e., a use
of m.
All other letters have only aesthetic purposes.
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A metatheorem.
We call a proposition negative if it has either ‘e’ or ‘o’ as
copula.
Theorem (Aristotle). If M is a mood with two negative
premises, then
BBCDF 6⊢ M.
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Metaproof (1).
Suppose o := ho1 , ..., on i is a BBCDF -proof of M .
The s-rules don’t change the copula, so if M has two negative premises, then so does
si (M ).
The superaltern of a negative proposition is negative and the superaltern of a positive
proposition is positive. Therefore, if M has two negative premises, then so does
pi (M ).
The m-rule and the per-rules don’t change the copula either, so if M has two negative
premises, then so do m(M ) and perπ (M ).
As a consequence, if o1 6= ci , then o(M ) has two negative
premisses.We check that none of Barbara, Celarent, Darii
and Ferio has two negative premisses, and are done, as o
cannot be a proof of M .
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Metaproof (2).
So, o1 = ci for either i = 1 or i = 2. By definition of ci , this means that the contradictory of
one of the premisses is the conclusion of o1 (M ). Since the premisses were negative, the
conclusion of o1 (M ) is positive. Since the other premiss of M is untouched by o1 , we have
that o1 (M ) has at least one negative premiss and a positive conclusion. The rest of the
proof ho2 , ..., on i may not contain any instances of ci .
Note that none of the rules s, p, m and per change the copula of the conclusion from positive
to negative.
So, o(M ) still has at least one negative premiss and a positive conclusion. Checking
Barbara, Celarent, Darii and Ferio again, we notice that none of them is of that form.
Therefore, o is not a BBCDF -proof of M . Contradiction.
q.e.d.
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Aristotelian modal logic.
Modalities.
Ap ≏ “p” (no modality, “assertoric”).
Np ≏ “necessarily p”.
Pp ≏ “possibly p” (equivalently, “not necessarily not p”).
Cp ≏ “contingently p” (equivalently, “not necessarily not
p and not necessarily p”).
Every (assertoric) mood p, q : r represents a modal mood
Ap, Aq : Ar. For each mood, we combinatorially have
43 = 64 modalizations, i.e., 256 × 64 = 16384 modal moods.
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Modal conversions.
Simple.
NXeY
NY eX
NXiY
NY iX
CXeY
CY eX
CXiY
CY iX
PXeY
PY eX
PXiY
PY iX
Accidental.
NXaY
NXiY
CXaY
CXiY
PXaY
PXiY
NXeY
NXoY
CXeY
CXoY
PXeY
PXoY
Relating to the symmetric
nature of contingency.
CXiY
CXeY
CXeY
CXiY
CXaY
CXoY
CXoY
CXaY
NXxY
AXxY
(Axiom T: ϕ → ϕ)
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Modal axioms.
What are the “perfect modal syllogisms”?
Valid assertoric syllogisms remain valid if N is added to
all three propositions.
Barbara (AaB, BaC:AaC)
NNN Barbara (NAaB, NBaC:NAaC).
First complications in the arguments for Bocardo and Baroco.
By our conversion rules, the following can be added to
valid assertoric syllogisms:
NNA,
NAA,
ANA.
Anything else is problematic.
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The “two Barbaras”.
NAN Barbara
ANN Barbara
NAaB
ABaC
NAaC
AAaB
NBaC
NAaC
From the modern point of view, both modal syllogisms are
invalid, yet Aristotle claims that NAN Barbara is valid, but
ANN Barbara is not.
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De dicto versus De re.
We interpreted NAaB as
“The statement ‘AaB’ is necessarily true.’
(De dicto interpretation of necessity.)
Alternatively, we could interpret NAaB de re (Becker 1933):
“Every B happens to be something which is necessarily an A.”
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Aristotelian temporal logic: the sea battle.
According to the square of oppositions, exactly one of “it is
the case that p” and “it is not the case that p” is true.
Either “it is the case that there will be a sea battle tomorrow”
or “it is not the case that there will be a sea battle
tomorrow”.
Problematic for existence of free will, and for Aristotelian
metaphysics.
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The Master argument.
Diodorus Cronus (IVth century BC).
Assume that p is not the case.
In the past, “It will be the case that p is not the case”
was true.
In the past, “It will be the case that p is not the case”
was necessarily true.
Therefore, in the past, “It will be the case that p” was
impossible.
Therefore, p is not possible.
Ergo: Everything that is possible is true.
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Megarians and Stoics.
Socrates (469-399 BC)
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++
Plato (c.427-347 BC)
Euclides (c.430-c.360 BC)
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++
Eubulides (IVth century)
Stilpo (c.380-c.300 BC)
Apollonius Cronus
Zeno of Citium (c.335-263 BC)
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Diodorus Cronus (IVth century)
Cleanthes of Assos (301-232 BC)
Chrysippus of Soli (c.280-207 BC)
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Eubulides.
Strongly opposed to Aristotle.
Source of the “seven Megarian paradoxes”, among
them the Liar.
The Liar is attributed to Epimenides the Cretan
(VIIth century BC); (Titus 1:12).
Aulus Gellius, Noctes Atticae.
Alessandro Garcea, Paradoxes in Aulus Gellius, Argumentation 17 (2003),
p. 87-98
Graham Priest, The Hooded Man, Journal of Philosophical Logic 31 (2002),
p. 445-467
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The seven Megarian paradoxes.
The Liar. “Is the man a liar who says that he tells lies?”
The concealed man. “Do you know this man who is concealed? If you do not, you do
not know your own father; for he it is who is concealed.”
The hooded man. “You say that you know your brother. Yet that man who just came in
with his head covered is your brother and you did not know him.”
Electra. “Electra sees Orestes : she knows that Orestes is her brother, but does not
know that the man she sees is Orestes; therefore she does know, and does not know,
her brother at the same time.”
The Sorites / the heap. “One grain of wheat does not make a heap. Adding one grain
of wheat doesn’t make a heap.”
The bald one. “Pulling one hair out of a man’s head will not make him bald, nor two,
nor three, and so on till every hair in his head is pulled out.”
The horned one. You have what you have not lost. You have not lost horns, therefore
you have horns.
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Quarternio terminorum.
I know my brother.
I do not know the hooded man.
The hooded man is my brother.
I do not know my brother.
know:
“I know a lot about X.”
know:
“I recognize X at sight
(regardless of whether X is hooded or not).”
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