Monthly Archives: April 2017

A Brief History of Validity #3

We read in my last post on this subject about the nineteen traditional valid forms of syllogisms named by medieval scholars, plus the five forms which can be deduced by subimplication of those with a universal conclusion. These comprise the twenty-four forms of syllogisms identified as valid in my Introductory Logic text.

Statements with Existential Import

However, since the time of George Boole, a 19th-century mathematician, only fifteen of those twenty-four forms are recognized as valid. Why is this? Boole argued that the truth of a particular statement cannot be inferred from the truth of its corresponding universal, because a particular statement asserts the existence of its subject, but a universal statement does not. That is, to say, “Some athletes are dedicated people” is to assert that at least one athlete exists, but to say that “All athletes are dedicated people” is only to say that if one is an athlete then one is a dedicated person. According to Boole, the four categorical statements should be interpreted this way:

All S is P = If S exists then it is P
No S is P = If S exists then it is not P
Some S is P = There exists at least one S that is P
Some S is not P = There exists at least one S that is not P

Particular statements are said to have existential import; they claim that the terms in the statement exist. Universal statements, however, do not have existential import; they are considered as material conditionals.

The Existential Fallacy

In this interpretation, no particular statement can be inferred from a universal statement, or from universal premises. One could not validly argue, for instance,

All grandfathers are fathers.
All fathers are men.
∴ Some men are grandfathers.

This AAI-4 (Bramantip) syllogism is said to make the existential fallacy, which is based on this sixth rule of validity: “A valid syllogism cannot have universal premises and a particular conclusion.” By the modern interpretation, the premises only say this:

If grandfathers exist then they are fathers.
If fathers exist then they are men.
∴ There exists at least one man who is a grandfather.

These premises do not claim that grandfathers or fathers or men exist, but the conclusion does. Thus the conclusion claims more than is contained within the premises, which means the syllogism is invalid.

There is much to commend the modern view of interpreting categorical statements. For example, in the traditional view, this is a valid chain of reasoning:

No athletes are people that breathe underwater ⇒ (by converse)
No people that breathe underwater are athletes ⇒ (by obverse)
All people that breathe underwater are non-athletes ⇒ (by subimplication)
Some people that breathe underwater are non-athletes.

Everyone would agree that the first statement is true, but most people would say that the last statement is false, because it seems to imply that there are people that breathe underwater.

Rethinking the Modern Interpretation

However, I think that the modern interpretation of categorical statements is potentially flawed. Does not the statement “No people that breathe underwater are athletes” seem to imply that there are people that breathe underwater? And why insist that particular statements have existential import? Consider these particular statements:

Some hobbits are not Shire dwellers.
Some black holes are members of binary stars.
Some of your sons will be the king’s horsemen.

Most people would argue that, in the sub-created world of Tolkien, the first statement is true, even though (in our world) hobbits do not exist. Most astronomers would argue that the second is almost certainly true, even though the existence of black holes is still in doubt. The last statement was uttered by Samuel to the people in the hope that it would not be true, that such sons would not exist.

Much more could be said, but the logic student should at least be aware that this debate exists. I would appreciate your thoughts.

Immediate Inference Cheat Sheet

Equivalent Immediate Inferences of the four Categorical Statements:

All S is P
=  No S is non-P  (obverse)
=  All non-P is non-S  (contrapositive)

No S is P
=  All S is non-P  (obverse)
=  No P is S  (converse)

Some S is P
= Some S is not non-P  (obverse)
= Some P is S  (converse)

Some S is not P
= Some S is non-P  (obverse)
= Some non-P is not non-S  (contrapositive)

Immediate inferences work in reverse:

All S is non-P
= No S is P  (obverse)

All non-S is non-P
= All P is S  (contrapositive)

No S is non-P
= All S is P  (obverse)

Some S is non-P
= Some S is not P  (obverse)

Some S is not non-P
= Some S is P  (obverse)

Some non-S is not non-P
= Some P is not S  (contrapositive)

Immediate inferences can be combined:

No non-S is P
= No P is non-S = All P is S  (converse, obverse)

Some non-S is P
= Some P is non-S = Some P is not S  (converse, obverse)

Other translations:

All non-S is P
= All non-P is S  (contrapositive)

No non-S is non-P
= All non-S is P  (obverse)

Some non-S is not P
= Some non-P is not S  (contrapositive)

Some non-S is non-P
= Some non-S is not P  (obverse)

All of this and more is included in this complete Immediate Inference Chart.

A Brief History of Validity #2

The 19 Traditional Forms

In the first post in this series, we saw that Aristotle identified 16 valid forms of categorical syllogisms (though he formally acknowledged only the first three figures). Some thirteenth-century logicians such as William of Sherwood and Peter of Spain recognized nineteen valid forms, giving them Latin names as a mnemonic device for ease of memorizing:

Barbara, Celarent, Darii, Ferioque prioris.
Cesare, Camestres, Festino, Baroco secundae.
Tertia Darapti, Disamis, Datisi, Felapton, Bocardo, Ferison habet.
Quarta insuper addit Bramantip, Camenes, Dimaris, Fesapo, Fresison.

The vowels in each name correspond with the mood, such that “Barbara” is AAA-1, “Cesare” is EAE-2, and so on. Thus the medievals recognized these valid forms:

Figure 1: AAA, EAE, AII, EIO
Figure 2: EAE, AEE, EIO, AOO
Figure 3: AAI, IAI, AII, EAO, OAO, EIO
Figure 4: AAI, AEE, IAI, EAO, EIO

The five forms not included in this list are AAI-1, EAO-1, EAO-2, AEO-2, and AEO-4. Why were these five not included? They are the forms in which the conclusion is the subimplication of moods with all universal statements, namely AAA-1, EAE-1, EAE-2, AEE-2,  and AEE-4. Thus they were seen as “weaker” forms of the syllogisms (why bother concluding the particular “Some S is not P”when you can conclude the universal “No S is P”?).

Defending the Missing Five

Interestingly, these five omitted forms can readily be shown to be equivalent to Bramantip (AAI-4) using immediate inferences, as follows:

AAI-4 (given)
All P is M
All M is S

∴ Some S is P

AAI-1 (taking the converse of the conclusion, correcting the premise order)
All M is S
All P is M

∴ Some P is S

EAO-1 (taking the obverse of the major premise and conclusion of the AAI-1)
No M is non-S
All P is M

∴ Some P is not non-S

EAO-2 (taking the converse of the major premise of the EAO-1)
No non-S is M
All P is M

∴ Some P is not non-S

AEO-2 (From the AAI-1, take the contrapositive of the major premise, obverse of the minor premise and conclusion)
All non-S is non-M
No P is non-M

∴ Some P is not non-S

AEO-4 (From the AEO-2, take the converse of the minor premise)
All non-S is non-M
No non-M is P

∴ Some P is not non-S.

This is one practical application of the immediate inferences learned in Lesson 27 of Introductory Logic.

A Brief History of Validity #1

Which forms of categorical syllogisms are valid? Logicians have disputed the answer for centuries, a dispute that can give us insight into the meaning of validity, the central concept of formal logic. This will be the first of a few posts in which I will briefly discuss the history of syllogistic validity.

Aristotle’s 16

It all started with Aristotle, who in his Prior Analytics, Book I, chapters 4-7, detailed sixteen valid forms:

Figure 1: AAA, EAE, AII, EIO
Figure 2: EAE, AEE, EIO, AOO
Figure 3: AAI, EAO, IAI, AII, OAO, EIO
Figure 4: EAO, EIO

If you read Prior Analytics (which is no trivial task), Aristotle presents only the first three figures as figures, omitting any mention of a fourth figure. But in chapter 7 he admits in passing the forms of EAO-4 and EIO-4 as valid, saying,

If A belongs to all or some B, and B belongs to no C … it is necessary that C does not belong to some A.

It is not difficult to see why Aristotle omits AAI-1, EAO-1, AEO-2, and EAO-2. These four forms are his AAA-1, EAE-1, AEE-2, and EAE-2 with the subimplication of the conclusion. Aristotle apparently saw no need to include syllogism forms with particular conclusions when the premises could imply the universal.

Aristotle and Figure 4

It is rather more difficult to understand why Aristotle does not admit the fourth figure, though logicians have argued that it has to do with how he defines a syllogism. We learn from Bertrand Russell, in his Cambridge Essays, that

The fourth figure…was added by Aristotle’s pupil Theophrastus and does not occur in Aristotle’s work, although there is evidence that Aristotle knew of fourth-figure syllogisms.

Theophrastus apparently recognized three more valid forms of figure 4: AAI, AEE, and IAI, bringing the total to 19. These were given Latin names by medieval scholars, but that will be the topic for my next post.

After Intermediate Logic?

What is recommended after Intermediate Logic? The short answer is: Rhetoric! But let me give you a bit more than that.

Introductory and Intermediate Logic together provide a complete foundational logic curriculum. Informal, categorical, and modern propositional logic are all included. The next step in a student’s classical education is to begin to apply what they have learned in logic to effective speaking and writing. This means that the student should move on to study formal rhetoric. Rhetoric applies the tools of logic: defining terms, declaring truth, arguing to valid conclusions, and refuting invalid ones. Indeed, of the modes of rhetorical persuasion – ethos, pathos, and logos – one-third is applied logic.

With this in mind, Roman Roads has released a new curriculum, Fitting Words: Classical Rhetoric for the Christian Student. I am the author of this text, and in Fitting Words I work to apply in rhetoric much of what the student has learned in logic. I am very excited about this project, because one significant reason that I wrote this text was to provide a satisfying answer the question of where to go next!

Take a look HERE for the most up-to-date information about Fitting Words.

Equivalence w/ Shorter Truth Tables

Mr. Nance,

Within Intermediate Logic Lesson 11, what would keep us from setting up the propositions both being true at the same time, and if there were a contradiction they would not be equivalent? Instead of setting them up one true and one false and if there’s a contradiction then they are equivalent?

That would be checking for consistency, not equivalence. If you set them both as true, and get a contradiction, then they are not consistent (which of course also means they are not equivalent, nor related by implication, per the chart in Introductory Logic, p. 71). But if you get no contradiction, all you have shown is that they can both be true, which is the meaning of consistency. To show equivalence, you have to show that they cannot have opposite truth values: the first cannot be true while the second is false, and vice versa.

Blessings!