Chapter 3 Hypothetical Syllogisms
Chapter 3
Hypothetical Syllogisms
As stated in Chapter 2, a hypothetical syllogism is a syllogism with at least one conditional premise, that is, at least one ―if…then…‖ premise. The ―If…then…‖ relationship may be expressed in ordinary language by using a number of different terms.
In checking hypothetical syllogisms expressed in ordinary language for their validity, it is therefore useful to be able to translate such conditional vocabulary into standard conditional form.
The Vocabulary of Conditional Statements
A conditional statement can be said to be in standard conditional form when it is stated with the antecedent stated first and the consequent stated second, and when the antecedent is preceded by the term ―If‖ and the consequent by the term ―then.‖ For example, the following statement is in standard conditional form:
If it rains then the roof will leak
In contrast, the following statement is not in standard conditional form:
The roof will leaks if it rains
The following table provides some common conditional vocabulary and illustrates how to translate it into standard conditional form:
Conditional
Vocabulary
Only if
Example
Standard Conditional Form
It rains only if there are clouds.
Only if there are clouds does it rain.
If it rains then there are clouds
1
If and only if
You are eligible to vote if and only if you have a valid voter registration card If you have a valid voter registration card then you are eligible to vote
&
If you are eligible to vote then you have a valid voter registration card
On condition that I will go on condition that you pay me. If I you pay me then I will go.
Entails,
Bachelorhood entails being unmarried. If one is a bachelor then one is unmarried Implies
Bachelorhood implies being unmarried In the event that
In the event that there’s a fire, take the stairs.
If there is a fire then take the stairs
When
When he lies, his left eye twitches.
If he lies then his left eye twitches
Where
Where there’s smoke,
Hypothetical Syllogisms
As stated in Chapter 2, a hypothetical syllogism is a syllogism with at least one conditional premise, that is, at least one ―if…then…‖ premise. The ―If…then…‖ relationship may be expressed in ordinary language by using a number of different terms.
In checking hypothetical syllogisms expressed in ordinary language for their validity, it is therefore useful to be able to translate such conditional vocabulary into standard conditional form.
The Vocabulary of Conditional Statements
A conditional statement can be said to be in standard conditional form when it is stated with the antecedent stated first and the consequent stated second, and when the antecedent is preceded by the term ―If‖ and the consequent by the term ―then.‖ For example, the following statement is in standard conditional form:
If it rains then the roof will leak
In contrast, the following statement is not in standard conditional form:
The roof will leaks if it rains
The following table provides some common conditional vocabulary and illustrates how to translate it into standard conditional form:
Conditional
Vocabulary
Only if
Example
Standard Conditional Form
It rains only if there are clouds.
Only if there are clouds does it rain.
If it rains then there are clouds
1
If and only if
You are eligible to vote if and only if you have a valid voter registration card If you have a valid voter registration card then you are eligible to vote
&
If you are eligible to vote then you have a valid voter registration card
On condition that I will go on condition that you pay me. If I you pay me then I will go.
Entails,
Bachelorhood entails being unmarried. If one is a bachelor then one is unmarried Implies
Bachelorhood implies being unmarried In the event that
In the event that there’s a fire, take the stairs.
If there is a fire then take the stairs
When
When he lies, his left eye twitches.
If he lies then his left eye twitches
Where
Where there’s smoke,