Categorical
Syllogisms
Now you know the basics about soundness, validity,
counterexample, and categorical syllogisms. You learned that, if an argument is
valid, there will be no case in which all the premises are true while
the conclusion is false. You also learned a basic method (counterexample)
for showing that an argument is invalid. Here you’ll learn another method for proving whether a given
categorical syllogism is valid or invalid. First, though, we have to learn a
little more about categorical syllogisms themselves.
All categorical syllogisms
consist of exactly three statements (or propositions): there are two
premises and one conclusion. Also, in a categorical syllogism, there are
exactly three terms (such as the terms A, B and C, below; or the terms
“cats”, “trees” and “dogs”, in the example below that):
All A are B
All B are C
All A are C
All oaks are trees*
All trees are plants
All oaks are plants
*This is not in the standard
form that we’ll discuss below.
You already know that arguments are made up of
statements. In categorical syllogisms, there
are only four kinds of statements. These are called standard form
categorical statements or propositions, as follows:
1) All S is/are P
2) No S is/are P
3) Some S is/are P
4) Some S is not/are not P
(Notice that a statement such as “All S is not P” is
not a possibility, but it is equivalent to “No S is P”.)
For logical purposes, “Some” means “at least one”.
Thus, the statement, “Some cats are felines” is true. You might think: “But
wait – ALL cats are felines! So how can it be true to say that ‘Some cats are
felines’? The answer is, “some” means at least one. So, both of the following
statements are true: “Some cats are felines”, and “All cats are felines”.
Similar considerations apply to the statement, “Some
cats are not dogs”. If at least one cat is not a dog, the statement is true;
so, of course, it is true. It’s also true that “No cats are dogs”.
Statements have parts: you will need to learn
them in order to apply the rules for categorical syllogisms. The parts are as
follows:
1) A quantifier (All, No or Some)
2) A subject term (“S” in the four statements
given above)
3) A predicate term (“P” in the four statements
given above)
4) A copula (is/are and is not/are not). The copula joins the subject term to the predicate
term.
So, for example, let’s look at the parts of the
following statement:
“No oaks are maples.”
 The quantifier is “No”
 The subject term is “oaks”
 The predicate term is
“maples”
 The copula is “are”
Here’s another example:
“Some mammals are not canines.”
 The quantifier is “Some”
 The subject term is
“mammals”
 The predicate term is
“canines”
 The copula is “are not”
Additionally, the four kinds of statements that make
up categorical syllogisms are said to have both quantity and quality.
There are two kinds of quantity: universal and particular. Also, there
are two kinds of quality: affirmative (it affirms something) and negative
(it denies something). Let’s see how these apply to the four kinds of
statements, below.
1) All S is/are P = Universal (because “all” is universal), Affirmative
(because it affirms something – that All S is/are P).
2) No S is/are P = Universal (because “no” is universal), Negative (because
it denies something).
3) Some S is/are P = Particular (because “some” is particular), Affirmative
(because it affirms something).
4) Some S is not/are not P = Particular, Negative.
Finally, for the sake of abbreviating these
statements, the following naming convention has been in effect for several
hundreds of years:
A = All S is/are P
E = No S is/are P
I = Some S is/are P
O = Some S is not/are not P
(For the future, I will drop the is/are
convention and use one or the other – but remember that either is fine.)
So, here is the complete table of information for the
four kinds of standard form statements that make up categorical syllogisms:
Statement 
Letter Name 
Quantity 
Quality 
All S are P 
A 
universal 
affirmative 
No S are P 
E 
universal 
negative 
Some S are P 
I 
particular 
affirmative 
Some S are not P 
O 
particular 
negative 
OK, now for a somewhat trickier concept: distribution.
For the moment, just accept and memorize the following, then I will attempt to
explain it:
“A term is said to be distributed if
it is either the subject of a universal or the predicate of a negative.”
Here is another table that reflects that additional
piece of information:
Statement 
Letter Name 
Quantity 
Quality 
Terms Distributed 
All S are P 
A 
universal 
affirmative 
S 
No S are P 
E 
universal 
negative 
S and P 
Some S are P 
I 
particular 
affirmative 
none 
Some S are not P 
O 
particular 
negative 
P 
So, you may be thinking: “Wonderful. What exactly is
distribution, and why should I care?” Good questions. You need to know how to
determine if a term is distributed when it comes to applying the rules for
categorical syllogisms. What does “distribution” mean? This is a little confusing, but
bear with me. I’m not making it confusing – it just is, and has
been, for countless generations of logic students. But here goes: When a term
is distributed in a statement, it means that the statement says something about
every member of the class that the term denotes.
So, in the “A”
statement, “All S is P”, the term “S” is distributed, because the statement
“All S is P” says something about every single S – namely, that they are all P.
In the “E”
statement, “No S are P”, both the S term and the P
term are distributed, since you are making a claim about all S’s, and all P’s.
Moving on to the “I”
statement, “Some S are P”, neither term is distributed, and that’s pretty easy
to see. You are not making a claim about all of S, nor are you making a claim
about all of P.
OK – so the last one is the toughest, and to many, it
never makes intuitive sense, you just have to accept that it is so, and if you
want to know more, take an advanced logic class. In the “O” statement, “Some S are are
not P”, the P term is distributed. Why? Because you have asserted something
about all of P – namely, that all of P falls outside at least one S. You don’t
like that, do you? No one does.
Next: Every syllogism has a major term, a minor
term, and a middle term, each of which appears exactly twice. The
major term, by definition, is the predicate of the conclusion. The minor term
is the subject of the conclusion. The middle term occurs once in each premise,
but not in the conclusion. Here is an illustration:
All felines are cats
No dogs are cats
No dogs are felines
 In this syllogism, the major term, or the predicate
of the conclusion, is felines.
 The minor term, or the subject of the conclusion, is
dogs.
 The middle term is cats.
( By the way, it is valid. )
Here is another illustration:
All horses are animals
Some dogs are not horses
Some dogs are not animals
 The major term is animals.
 The minor term is dogs.
 The middle term is horses.
( By the way, it is invalid.)
There are 1letter abbreviations for the major term,
minor term, and middle term. They are:
P = major term
(because it is the predicate of the conclusion)
S = minor term
(because it is the subject of the conclusion)
M = middle term
(“M” for middle term)
Also, we distinguish the two different premises in a
categorical syllogism according to whether they contain the major term or the
minor term. Quite simply, the major premise contains the major term, the minor premise contains the minor term.
Finally, just as a categorical statement or
proposition (A, E, I or O) is said to be in standard form, a categorical
syllogism has a standard form, too. In order for a categorical syllogism to be
in standard form, the following requirements must be met:
1) All three statements are standard form categorical
propositions (A, E, I or O).
2) The two occurrences of each term (major, minor and
middle) are identical, and are used in the same sense throughout the argument.
3) The major premise is listed first, the
minor premise second, and the conclusion last.
Here, for example, are three categorical syllogisms in
standard form:
1)
All P are M
Some S are not M
No S are P
2)
All M are P
All S are M
All S are P
3)
All felines are mammals
All cats are felines
All cats are mammals
Here are two that are NOT in standard form, because
the minor premise is listed first:
4)
All S are M
All M are P
All S are P
(It is still valid, but it is not in standard form.)
All cats are felines
All felines are mammals
All cats are mammals
(Again, still valid, but not in
standard form.)
It may surprise you to know that categorical
syllogisms have both moods and figures. To determine these, you
first need to be sure that your categorical syllogism is in standard form.
Then, the mood is simply the letter names of the categorical statements, in
order, from the first premise, to the second premise, then the conclusion. It’s
easier to see this by an example:
1) The mood below is AEO:
All P are M (A)
No S are
M (E)
Some S are
not P (O)
2) The mood below is IAE:
Some M are
P (I)
All M are S (A)
No S are
P (E)
3) The mood below is AAO:
All P are M
All M are S
Some S are
not P
So much for mood. Now, you may be wondering, what determines the order
in which the middle term occurs in each premise? Does it come first or second?
The placement of the middle term is determined by the figure of the
categorical syllogism. Since there are only four possible placements, there are
only four possible figures. What are the four possible placements? Think about
it – in each premise the middle term could be either first or second. So, the
four possibilities are:
Figure 1

Figure 2

Figure 3

Figure 4

First premise: Middle term first 
First premise: Middle term second 
First premise: Middle term first 
First premise: Middle term second 
Second premise: Middle term second 
Second premise: Middle term second 
Second premise: Middle term first 
Second premise: Middle term first 
These four figures were originally devised by
Aristotle, and have been memorized in exactly this order ever since. So, join
two thousand years’ worth of logic students in remembering your four figures!
Here is a mnemonic that many have found useful in trying to remember which figure
is which:
The first figure sort of looks like
an “S”, the last a “Z”, with two backtoback “C’s” in the middle. How does it work? The horizontal lines represent the
premises, and the middle term falls where the lines intersect. So, in the first
figure, which looks like an “S”, the middle term comes first in the first
premise (because that’s where the lines intersect), then it comes second in the
second premise (again, because that’s where the lines intersect). In the second
figure, which looks like a backward “C”, the middle term comes second in both
premises, and so forth, as per the table above.
By the way, it may have occurred to you by now that
there are a finite number of standard form categorical syllogisms. How many
do you think there are?
It may also have occurred to you that of this finite
number of standard form categorical syllogisms, only a certain number of them
are valid. You’ll soon be able to tell
for yourself.
Next – some practice on mood and figure. Remember –
you must make sure the syllogism is in standard categorical form, or the mood
and figure may be wrong! Be sure, for example, that the major premise (the
one containing the major term) is listed first. Here are some sample moods
and figures for you to study:
1) Mood AAA, Figure 1:
All M are P
All S are M
All S are P
 or –
All monkeys are primates
All spider monkeys are monkeys
All spider monkeys are primates
2) Mood AAA, Figure 2:
All P are M
All S are M
All S are P
 or –
All pandas are mammals
All sloths are mammals
All sloths are pandas
3) Mood AAA, Figure 3:
All M are P
All M are S
All S are P
 or 
All mammals are pandas
All mammals are sloths
All sloths are pandas
4) Mood AAA, Figure 4:
All P are M
All M are S
All S are P
 or –
All poodles are mammals
All mammals are sentient beings
All sentient beings are poodles
Now, let’s practice
identifying the mood and figure of an existing standard form categorical
syllogism.
1) Identify the mood and figure:
No P are M
Some S are not M
All S are P
2) Identify the mood and figure:
Some M are P
Some S are not M
Some S are not P
3) Identify the mood and figure:
No M are P
No M are S
All S are P
4) Identify the mood and figure:
All P are M
No M are S
Some S are not P
5) Identify the mood and figure:
All S are M
No M are P
All S are P
(Hint: Be careful – remember standard form!)
Answers
1) Mood: EOA Figure 2 (Usually just written together
as EOA2)
No P are M
Some S are not M
All S are P
2) IOO  1
Some M are P
Some S are not M
Some S are not P
3) EEA  3
No M are P
No M are S
All S are P
4) AEO  4
All P are M
No M are S
Some S are not P
5) Be careful – this syllogism is not written in
standard form:
All S are M
No M are P
All S are P
Before you determine the mood and figure, you have to
switch the order of the premises so that the major premise is listed first:
No M are P
All S are M
All S are P
The mood is EAA, and the figure is 1
OK, now we can finally move on to the muchanticipated
rules for categorical syllogisms. First, I’ll list them, then
discuss them further.
Rules for Categorical
Syllogisms!
Rule 1: The
middle term must be distributed at least once.
Rule 2: If a
term is distributed in the conclusion, then it must be
distributed in the premise.
Rule 3: Two
negative premises are not allowed.
Rule 4: A
negative premise requires a negative conclusion, and a negative conclusion
requires a negative premise.
Rule 5: If
both premises are universal, the conclusion cannot be particular.
Note: If
only rule 5 is broken, the syllogism is said to be conditionally valid –
that is, valid on the condition that certain terms actually denote things that
are real, or that exist.
If no rules are broken, the syllogism is valid.
Otherwise, it is invalid. (I’ll address the exception concerning rule 5,
below).
Let’s look at some examples.
Here is the table again from above:
Statement 
Letter Name 
Quantity 
Quality 
Terms Distributed 
All S are P 
A 
universal 
affirmative 
S 
No S are P 
E 
universal 
negative 
S and P 
Some S are P 
I 
particular 
affirmative 
none 
Some S are not P 
O 
particular 
negative 
P 
Rule 1: The middle term must be distributed at least
once. (Now you understand why it was
so important to learn about distribution!) Let’s see how this rule works with
the following syllogism:
All sharks are fish
All salmon are fish
All salmon are sharks
First, identify the middle term, which is “fish”. Then,
check to see if it is distributed in either premise. You can see that it is not
distributed in the first premise, because it is neither the subject of a
universal, nor the predicate of a negative (in fact, it is the predicate of a
universal affirmative). So, now check the second premise. The term “fish” is
not distributed here either, for the same reason. So, this rule is violated,
and the syllogism is invalid.
Let’s see another example that violates Rule 1:
Some cats are pets
Some pets are dogs
Some dogs are cats
First, identify the middle term, which is “pets”. It
is not distributed in the first premise, because the first premise is an “I”
proposition that distributes no terms. The same is true for the second premise,
so the middle term, “pets”, is not distributed at all. The rule is violated,
and the syllogism is invalid. By the way, you can see that no categorical
syllogism that has two “I” statements for premises could ever be valid, because
the middle term could never be distributed. In other words, IIA, IIE, IIO and
III could never be valid, no matter what figure they’re in.
Rule 2: If a term is distributed in the conclusion,
then it must be distributed in the premise. Here is a syllogism that violates this rule:
All squirrels are animals
Some wombats are not squirrels
Some wombats are not animals
First, check to see if a term (either the subject term
or the predicate term) is distributed in the conclusion. The subject term,
“wombats”, is not distributed, but the predicate term, “animals”, is (because
it is the predicate of a negative). Next, check to see if the term “animals” is
distributed in the premise in which it occurs, which is the statement, “All
squirrels are animals”. In this statement, the term “animals” is not distributed,
because it is neither the subject of a universal nor the predicate of a
negative. So, this rule is violated, and the syllogism is invalid.
Let’s look at a second example that violates Rule 2:
All tigers are mammals
All mammals are animals
All animals are tigers
First, check to see if a term is distributed in the
conclusion. In this case, the term “animals” is the subject of a universal, so
it is distributed, but the term “tigers” is not distributed. OK, so now check
to see if the term “animals” is distributed in its premise, which reads, “All
mammals are animals”. The A proposition, as you know, does not distribute the
predicate term, so the term “animals” is not distributed in the premise, and
the rule is violated. This syllogism is invalid.
Let’s do one more example of a syllogism that violates
Rule 2:
Some creatures are camels
Some creatures are not leopards
No leopards are camels
First, check the conclusion to see if any terms are
distributed. In this case, the conclusion is an “E” statement, so both terms,
“leopards” and “camels” are distributed. (The conclusion is a universal
negative, so “Leopards” is the subject of a universal, and “camels” is the
predicate of a negative). Now, we have to check to see if they are both
distributed. Is the term “leopards” distributed in the premise that reads,
“Some creatures are not leopards”? Yes, it is, because it is the predicate of a
negative. So far, so good. Now, let’s check the term
“camels”. Is it distributed in the premise that reads, “Some creatures are
camels”? No, an “I” statement does not distribute any terms. So, the rule is
violated, because “camels” is distributed in the conclusion, but not in the
premise. The syllogism is invalid.
Rule 3: Two negative premises are not allowed. This rule is probably the easiest to check. What is a
negative premise? One that is either an “E” statement (No S are P) or an “O”
statement (Some S are not P). So, if you have EE, OO, EO, or OE as your two
premises the syllogism is invalid. Let’s look at some examples:
No fish are mammals
Some dogs are not fish
Some dogs are mammals
The first premise is negative (an “E” statement), and
so is the second (an “O” statement), so both premises are negative, and the
rule is violated. The syllogism is invalid.
Here’s a second example:
No bricks are marshmallows
No marshmallow are thimbles
Some thimbles are bricks
The first and second premises are both “E” statements,
so they are both negative, and the rule is violated. The syllogism is invalid.
Rule 4: A negative premise requires a negative
conclusion, and a negative conclusion requires a negative premise. Let’s look at an example:
All ducks are birds
Some elephants are not ducks
Some elephants are birds
First, check to see if you have a negative premise. In
this case, we do, “Some elephants are not ducks”. Then, check to see if the
conclusion is negative. In this case, “Some elephants are birds” is not
negative, so the first part of this rule is violated, and we know the syllogism
is invalid.
Here’s another example:
All circles are shapes
All squares are shapes
Some squares are not circles
First, check to see if you have a negative premise. In
this case, we do not. Next, for the second part of the rule, see if the
conclusion is negative. In this case, it is negative. So, since the conclusion
is negative, we need to see if one of the premises is negative, too. As we’ve
already seen, neither premise is negative, so the second part of the rule is
violated, and the syllogism is invalid.
Rule 5: If both premises are universal, the conclusion
cannot be particular.
Note: If
only rule 5 is broken, the syllogism is said to be conditionally valid –
that is, valid on the condition that certain terms actually denote things that
are real, or that exist.
First, let’s just see this rule works, and then I’ll
explain the point about conditional validity. Here is an example of a syllogism
that violates Rule 5:
All mammals are animals
All unicorns are mammals
Some unicorns are animals
Both premises are universal, but the conclusion is
particular, so the rule is violated. The syllogism is invalid. But only Rule 5
is violated, so doesn’t that mean it’s conditionally valid? Yes. But what does
that mean? The condition it refers to is the condition that certain terms
actually exist. When Aristotle invented this logic, he wanted to use it to talk
about things in the world – things that really existed – so that assumption was
built in. More modern logics, however, need to be able to talk about things
that may or may not exist, and it can make a difference. In the argument above,
the argument is conditionally valid – it is valid on the condition that
unicorns actually exist. But, of course, they do not. So, the argument is
invalid.
A fuller treatment of this problem, called “the
problem of existential import”, is beyond the scope of this course. I just
wanted you to have a basic explanation for the sake of completing the rules.
Now for the good news: I won’t require you to learn
the difference between syllogisms that are conditionally valid and ones that
are unconditionally valid. For testing purposes, the syllogisms will either be
valid (unconditionally) or invalid. I won’t give you test questions in which only
Rule 5 is violated, though I may give you some which violate Rule 5 along with
one or more other rules.
Practice Problems
Reconstruct the following syllogisms and indicate any
and all rules for categorical syllogisms that it violates. Indicate if it is
valid or invalid.
1) I I I
– 4
2) IAO – 3
3) EAA  1
4) AEE – 4
5) AOO – 2
Answers:
1) I I I
 4
Some P are M
Some M are S
Some S are P
Invalid, violates Rule 1 (middle term is not
distributed).
2) IAO  3
Some M are P
All M are S
Some S are not P
Invalid, violates Rule 2 and Rule 4.
3) EAA  1
No M are P
All S are M
All S are P
Invalid, violates Rule 4
4) AEE – 4
All P are M
No M are S
No S are P
Valid, no rules broken.
5) AOO – 2
All P are M
Some S are not M
Some S are not P
Valid, no rules broken.