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Slides on conditionals, disjunction, validity, soundness, modus ponens, modus tollens, chain argument, disjunctive syllogism, and dilemma
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A Standard of Critical Thinking
Soundness
Review
Logical conjunctions, often expressed in English by ‘and’, is true when the component claims it joins are true, otherwise it is false. It is symbolized by ‘&’. It’s logical form is P & Q.
Logical negation, often expressed in English by ‘not’, is true when the component claim is false, false when the component claim is true. It is symbolized by ‘~’ and has the logical form ~P.
Review
The Principle of Noncontradiction, states that no thing can, at the same time and in the same manner, both have and not have the same property.
Contradiction, a special form of conjunction in which a claim and its negation are joined—they are always false. The logical form of a contradiction is P & ~P.
Review
The Standard of Consistency—accept only those beliefs which are consistent with each other and any accessible evidence.
Reductio ad ridiculum, appealing to ridicule (making fun of an opposing view) rather than providing reasons against it—it is a fallacy.
Equivocation, to use a term ambiguously or vaguely in an argument—it is a fallacy.
Reviewdouble negation—any even number of negations cancel each other out.to prove a conjunction false prove that one of the component claims is false.to evaluate by
contradiction—isolate the subject and predicate, generate lists of things that fall under each, stopping when you determine that they are not identical.
proof by counterexample—Choose an item that is not on both lists, explain how the definition says it should be, then explain why it is not, indicate the inconsistency, and reject or revise the definition.
Line of Reasoning
An explanation showing that the
definition should be true of a specific example (thing or
event).
Another explanation showing that the
definition is not true of the same example.
Reject the original
definitionOriginal definition.
A Method for Reasoning with Contradictions
Proof by Counterexample
Another Line of Reasoning
Line of Reasoning
2. reasons3. conclusion
4. other reasons5. other conclusion
6. P & ~P
1. Claim
Reductio ad absurdamIndirect Proof, Proof by
Counterexample
Reductio ad absurdam
Another Line of Reasoning
7. Rejection
The Logical Form of a Reductio
Reductio ad absurdam, Indirect Proof, Proof by
Counterexample1.claim2.reasons3.conclusion4.other reasons5.other conclusion6.contradiction
7.rejection
Reviewto avoid equivocating—define key terms by giving them one (to disambiguate) clear (to avoid vagueness) meaning.
to avoid equivocating—use the Principle of Charity to settle on the best interpretation, whether normative or descriptive.
to avoid reductio ad ridiculums—use the Principle of Sufficient Reason and attempt to provide reasons for each claim.
Logical Complexity
The Conditional
A Logically Simple Truth
Sarah attends Stanford.
1 True
2 False
Two logical possibilities
Given that we’ve filled
in the indices,
made the ceteris paribus
explicit, and defined key
terms.
Combining Logically Simple Truths
Sarah attends Stanford.
Sarah goes into debt.
1 True True
2 True False
3 False True
4 False False
Four states of affairs
(states) or possible worlds
Logical Conjunction
Sarah attends Stanford
ANDSarah goes into
debt
1 True True True
2 True False False
3 False False True
4 False False False
Logical Conditional
IFSarah attends
StanfordTHE
NSarah goes into
debt
1 True ? True
2 True ? False
3 False ? True
4 False ? False
Logical Conditional
IFSarah attends
StanfordTHE
NSarah goes into
debt
1 True True True
2 True ? False
3 False ? True
4 False ? False
Logical Conditional
IFSarah attends
StanfordTHE
NSarah goes into
debt
1 True True True
2 True False False
3 False ? True
4 False ? False
Logical Conditional
IFSarah attends
StanfordTHE
NSarah goes into
debt
1 True True True
2 True False False
3 False True True
4 False True False
Logical Conditional
IFSarah attends
StanfordTHE
NSarah goes into
debt
1 True True True
2 True False False
3 False True True
4 False True False
Taking ‘if’ seriously
Logical Conditional
If Sarah goes to Stanford then she will incur debt.
If Sarah will incur debt then she goes to Stanford.
≠
Either Sarah will incur debt or she goes the Stanford.
Either Sarah goes the Stanford or she will incur debt.
=
Both Sarah will incur debt and she goes the Stanford.
Both Sarah goes the Stanford and she will incur debt.
=
Order Matters
Logical Conditional
If Sarah goes to Stanford then she will incur debt.
Antecedent Consequent
The Case of Iffy AdviceSarah Scatterleigh weighed her options. She could transfer to Stanford, which had a stronger program for her major and a better track record of placing graduates into the job market. But Stanford cost quite a bit more than the school she was presently attending, Jefferson University. She sought advice from her friend, Johnny Nogginhead, musing that If I go to Stanford then I’ll go into debt.But, replied Johnny, You don’t go to Stanford.I know, said Sarah, I said If I go to Stanford….But you don’t, retorted Johnny, you go to Jefferson!I never said I didn’t, said an exasperated Sarah, I know I don’t go to Stanford, my point is that going to Stanford might mean going into debt.Why didn’t you just say that, said Johnny.
In a certain sense, ‘if’ means the antecedent isn’t true.
Logical Interpretations of ‘if’
If Sarah goes to Stanford then she will incur debt.
Either Sarah doesn’t go to Stanford OR she does AND will incur debt.
It is NOT that Sarah could go to Stanford and not incur debt.
Logical Interpretations of ‘if’
IF Sarah attends Stanford THEN Sarah goes into debt
1 True True True
2 True False False
3 False True True
4 False True False
Either
Sarah doesn’t attend Stanford
OR(she does
ANDgoes into
debt)
1 False True True True True
2 False False True False False
3 True True False False True
4 True True False False False
It’s NOT that
(Sarah attends Stanford
AND doesn't go into debt)
1 True True False False
2 False True True True
3 True False False False
4 True False False True
Truth Values match line for
line (across
all possible worlds)
Logical conditional, often expressed in English by ‘if…then….’, is true when the antecedent is true and the consequent is false, otherwise it is true. It is symbolized by ‘⊃’. It’s logical form is P ⊃ Q.
Logical Form of Conditionals
IF P THEN QP ⊃ QP → Q
P ⊃ Q
1 TrueTrue
True
2 TrueFals
eFalse
3 FalseTrue
True
4 FalseTrue
False
to prove a conditional false
Prove that the antecedent is true while the consequent is false.
to interpret conditionals logically
translate it as ‘Either not p or (p and q)’ or ‘It not that (p and not q)
indicators for conditionals
a. if p then qb. q if pc. p only if qd. not p unless qe. supposing p, q
f. imagine p ... qg. assuming p, qh. all p are qi. whenever p, qj. when p, q
A Standard of Critical Thinking
Validity
Argument, a set of claims in which some claims (premises) are offered to show the truth (or falsehood) of another claim (the conclusion). A line of reasoning.
Arguments
Lines of reasoning
If it is red then it has color,if it has color then it emits or reflects a wavelength of light,thus if it is red then it emits or reflects a wavelength of light.
When a government abuses rights it ought to be removed.The king abuses rights and so he ought to be removed.
If it is a mammal then it gives live birth.It lays eggs.So it’s not a mammal.
If living pigeons didn’t all come from rock pigeons then they must have come from other kinds of pigeons.There are no other kinds of pigeons.This established they all come from rock pigeons.
If anything is a dog then it is a mammal.If anything is a mammal then it is an animal.which proves that if anything is a dog then it is an animal.
When water is heated to 212° it boils.It’s not boiling, which demonstrates it hasn’t been heated to 212°.
All dogs are mammals.All mammals are animals.Hence all dogs are animals.
indicators for premises
a.asb.as shown byc.becaused.deduce frome.derive fromf. finally, the last reasong.first, second, third,…
nexth.follows fromi. forj. inasmuch as
k.indicated byl. is the reason thatm.it is the case thatn.may be deduced fromo.may be derived fromp.may be inferred fromq.one reason being…r. sinces. the fact thatt. the reason
indicators for conclusions
a.as a resultb.consequentlyc.demonstratesd.entailse.establishesf. henceg.I conclude thath.impliesi. in conclusionj. infer
k.it follows thatl. justifies m.meansn.proveso.showsp.soq.thenr. therefores. thus
Sets of Claims
Are Without Any Particular Order
Today is either Tuesday or Thursday.
She has chemistry today. She will recognize her teacher.
She will recognize her classmates.
The class meets in the same room.
She’s not dreaming.
The alarm did not go off.
Today is Monday.
Sarah’s beliefs
She went to the right room.
She is late.
Arguments
Have Order (‘∴’ means therefore)
1.If it is red then it has color,2.if it has color then it emits or reflects a wavelength of
light,3. ∴ If it is red then it emits or reflects a wavelength of
light.
1.When a government abuses rights it ought to be removed.
2.The king abuses rights and 3. ∴ He ought to be removed.
1.If it is a mammal then it gives live birth.
2.It lays eggs.3. ∴ It’s not a mammal.
1.If living pigeons didn’t all come from rock pigeons then they must have come from other kinds of pigeons.
2.There are no other kinds of pigeons.
3. ∴ They all come from rock pigeons.
1. If anything is a dog then it is a mammal.
2. If anything is a mammal then it is an animal.
3. ∴ If anything is a dog then it is an animal.
1.When water is heated to 212° it boils.
2.It’s not boiling, 3. ∴ It hasn’t been heated to 212°.
1. All dogs are mammals.2. All mammals are animals.3. ∴ All dogs are animals.
Validity, if the premises are true then the conclusion is true.
Validity, either the premises are false, or they are true and so it the conclusion.
…it is not possible that the premises are true while the conclusion is false.
Another Emergent Property
Wetness emerges as a property of water when hydrogen and oxygen are properly combined—though neither are wet themselves. In a similar manner, validity emerges when claims are properly structured into an argument.
Validity
Validity
Versus Truth
Validity Truth
Applies to whole arguments
Applies to claims, both simple and
complex
Does not apply to claims
Does not apply to arguments
As Technical Terms
Valid Arguments
1If anything is a dog then it is mammal.
2If anything is a mammal then it is an animal.
∴3If anything is a dog then it is an animal.
Premises
Conclusion
✔ 1If anything is a dog then it is mammal.
✔ 2If anything is a mammal then it is an animal.
✔ ∴3If anything is a dog then it is an animal.
animals
mammals
dogs
��
��
��
��
��
��
��
����
��
Here, by premise 1, no dog can be at the bottom of the blue (it is outside of mammals). By premise 2 no mammal can be at the bottom of the green (it is outside of the animals). So there is no place left for a dog to be.
Valid Arguments
✔ 1If anything is a dog then it is mammal.
✔ 2If anything is a mammal then it is an animal.
✔ ∴3If anything is a dog then it is an animal.
So this argument is valid
Valid Arguments
1 If anything is a hectagon then it has more sides than a chiliogon.
2 If anything is a chiliogon then it has more sides than a megagon.
∴3 If anything is a hectagon then it has more sides than a megagon.
Premises
Conclusion
If:hectagon means 1,000,000 sided, and;chiliogon means 1,000 sided, and;megagon means 100 sided;then the conclusion would have to be true.
Valid Arguments
1 If anything is a hectagon then it has more sides than a chiliogon.
2 If anything is a chiliogon then it has more sides than a megagon.
∴3 If anything is a hectagon then it has more sides than a megagon.
So this argument is valid
Valid Arguments
But:megagon means 1,000,000 sided, andhectagon means 100 sided (chiliogon does mean 1,000 sided)so the premises are in fact false.
✘ 1 If anything is a hectagon then it has more sides than a chiliogon.
✘ 2 If anything is a chiliogon then it has more sides than a megagon.
∴3 If anything is a hectagon then it has more sides than a megagon.
Valid Arguments
✘ 1 If anything is a hectagon then it has more sides than a chiliogon.
✘ 2 If anything is a chiliogon then it has more sides than a megagon.
∴3 If anything is a hectagon then it has more sides than a megagon.
But this argument is valid, because if the premises were true then the conclusion
would be true too.
Valid Arguments
✘ 1 If anything is a hectagon then it has more sides than a chiliogon.
✘ 2 If anything is a chiliogon then it has more sides than a megagon.
∴3 If anything is a hectagon then it has more sides than a megagon.
Test it by replacing ‘hectagon’, ‘chiliogon’ and ‘megagon’ with ‘triangle’, rectangle’,
and ‘octogon’.
Valid Arguments
Valid Arguments
Both Arguments have the Form:
1.P ⊃ Q2.Q ⊃ R3. ∴ P ⊃ R
✔ 1 If anything is a camel then it has four legs.
✔ 2 If anything is a pig then it has four legs.
✘ ∴3 If anything is a pig then it is a camel.
This argument is invalid, because even if the premises are true then the conclusion is
not.
Invalid Arguments
This Argument Has the Form:
Invalid Arguments
P ⊃ RQ ⊃ R
∴ P ⊃ Q
It is not a valid form.
to determine validity
check to see if the form of the argument fits one of the valid patterns.
Yes If it is…
If it is not…
…then it is invalid
Is it possible for the premises to be true while the conclusion is false?
Validity: The Test
No …then it is valid
Yes If it does…
If it does not…
…then it is valid
Does the argument have a valid form?
Validity: A Quick Check
No …then it is invalid
A Complex Standard of Critical Thinking
Soundness
Soundness, valid arguments with true premises.
argumentsA Taxonomy
Arguments
valid arguments
sound arguments
1. If anything is a dog then it is has four legs.
2. If anything is a cat then it is has four legs.
3. ∴ If anything is a dog then it is a cat.
1. If anything is a pig then it is a quadruped.
2. Trakr is a quadruped.3. ∴ Trakr is a pig.
1. If anything is a camel then it has four legs.
2. If anything is a pig then it has four legs.
3. ∴ If anything is a pig then it is a camel.
1. If anything is a hectagon then it has more sides than a chiliogon.
2. If anything is a chiliogon then it has more sides than a megagon.
3. ∴ If anything is a hectagon then it has more sides than a megagon.
1. If anything is a dog then it is mammal.2. If anything is a mammal then it is an
animal.3. ∴ If anything is a dog then it is an
animal.
1. If anything is a camel then it is a has humps.
2. Thor has no humps.3. ∴ Thor is not a camel.
1. If anything is wild then it is free.
2. Peter is not wild.3. ∴ Peter is not free.
Yes Are the premises true?
it is sound
Is the argument valid?
Soundness
it is unsound
Yes
No
No
to determine soundness
check to see if the form of the argument fits one of the valid patterns, then check to see if the premises are true.
Some Common Forms
Validity
Chain Arguments
1. If anything is a dog then it is mammal.2. If anything is a mammal then it is an
animal.3. ∴ If anything is a dog then it is an
animal.
Modus Ponens
Modus Tollens
1. When a government abuses rights it ought to be removed.
2. The king abuses rights .3. ∴ He ought to be removed.
1. If it is a mammal then it gives live birth.
2. It lays eggs.3. ∴ It’s not a mammal.
A Common Form
Chain Argument
Chain Arguments
1.If anything is a dog then it is mammal.
2.If anything is a mammal then it is an animal.
3. ∴ If anything is a dog then it is an animal.
The Parts of a Chain Argument
Chain Argument
If anything is a mammal then it is an animal.
If anything is a dog then it is mammal.
∴ If anything is a dog then it is an animal.
1.Conditional Premise2.Conditional Premise3.Conditional Conclusion
The Structure of a Chain Argument
Chain Argument
1.If anything is a dog then it is a mammal.2.If anything is a mammal then it is an animal.
3. ∴ If anything is a dog then it is an animal.
The conclusion has the same antecedent as the first premise…
The conclusion has the same consequent as the last premise…
The antecedents and consequents of the premises link up as in a chain.
Chain Arguments
Also called ‘hypothetical syllogisms’
1.P ⊃ Q2.Q ⊃ R3. ∴ P ⊃ R
Chain Arguments
Can have indefinitely many premises
1. P ⊃ Q2. Q ⊃ R3. R ⊃ S4. S ⊃ T5. ∴ P ⊃ T
to calculate the number of possible worlds
raise two to the power of the number of claims being evaluated, here there are three: P, Q, & R
23 = 2 • 2 • 2 = 8
Proving Chain Arguments Valid
(P ⊃ Q) (Q ⊃ R) (P ⊃ R)
1 True True True True True True
2 True True TrueFals
eTrue
False
3 TrueFals
eFals
eTrue True True
4 TrueFals
eFals
eFals
eTrue
False
5Fals
eTrue True True
False
True
6Fals
eTrue True
False
False
False
7Fals
eFals
eFals
eTrue
False
True
8Fals
eFals
eFals
eFals
eFals
eFals
e
Step One: Assign values to the simplest atomic claims, P, Q, & R
P PQ Q R R
(P ⊃ Q) (Q ⊃ R) (P ⊃ R)
1 True True True True True True True True True
2 True True True TrueFals
eFals
eTrue
False
False
3 TrueFals
eFals
eFals
eTrue True True True True
4 TrueFals
eFals
eFals
eTrue
False
TrueFals
eFals
e
5Fals
eTrue True True True True
False
True True
6Fals
eTrue True True
False
False
False
TrueFals
e
7Fals
eTrue
False
False
True TrueFals
eTrue True
8Fals
eTrue
False
False
TrueFals
eFals
eTrue
False
Step Two: Determine the values of the next simplest or molecular claims.
Proving Chain Arguments Valid
(P ⊃ Q) (Q ⊃ R) (P ⊃ R)
1 True True True✔True True True✔True True True
2 True True True TrueFals
eFals
eTrue
False
False
3 TrueFals
eFals
eFals
eTrue True True True True
4 TrueFals
eFals
eFals
eTrue
False
TrueFals
eFals
e
5Fals
eTrue True✔True True True✔
False
True True
6Fals
eTrue True True
False
False
False
TrueFals
e
7Fals
eTrue
False ✔
False
True True✔Fals
eTrue True
8Fals
eTrue
False ✔
False
TrueFals
e ✔Fals
eTrue
False
Step Three: Determine if there is a possible world where the premises are both true while the conclusion is false.
Proving Chain Arguments Valid
(P ⊃ Q) (Q ⊃ R) (P ⊃ R)
1 True True True✔True True True✔True True True
2 True True True TrueFals
eFals
eTrue
False
False
3 TrueFals
eFals
eFals
eTrue True True True True
4 TrueFals
eFals
eFals
eTrue
False
TrueFals
eFals
e
5Fals
eTrue True✔True True True✔
False
True True
6Fals
eTrue True True
False
False
False
TrueFals
e
7Fals
eTrue
False ✔
False
True True✔Fals
eTrue True
8Fals
eTrue
False ✔
False
TrueFals
e ✔Fals
eTrue
False
There is no possible world where the premises are true while the conclusion is false.
So Chain Arguments are valid.
Proving Chain Arguments Valid
An Unnamed Fallacy
(P ⊃ R) (Q ⊃ R) (P ⊃ Q)
1 True True True✔True True True✔True True True
2 TrueFals
eFals
eTrue
False
False
True True True
3 True True True✔Fals
eTrue True ✘ True
False
False
4 TrueFals
eFals
eFals
eTrue
False
TrueFals
eFals
e
5Fals
eTrue True✔True True True✔
False
True True
6Fals
eTrue
False
TrueFals
eFals
eFals
eTrue True
7Fals
eTrue True✔
False
True True✔Fals
eTrue
False
8Fals
eTrue
False ✔
False
TrueFals
e ✔Fals
eTrue
False
There is a possible world where the premises are true while the conclusion is false.
So arguments of this form are invalid.
A Common Form
Modus Ponens
modus ponens
1.When a government abuses rights it ought to be removed.
2.The king abuses rights .3. ∴ He ought to be removed.
The Parts of a Modus Ponens Argument
Modus Ponens
The king abuses rights .
When a government abuses rights it ought to be removed.
∴ He ought to be removed.
1.Conditional Premise2.Premise Affirming the Antecedent of the Conditional
3.Concluding the Consequent of the Conditional
The Structure of a Modus Ponens Argument
Modus Ponens
1.When a government abuses rights it ought to be removed.
2.The king abuses rights .3. ∴ He ought to be removed.
A conditional premise.
The conclusion is the consequent of the conditional premise.
A premise which affirms the antecedent of the conditional premise.
Modus Ponens
Also called ‘Affirming the Antecedent’ and ‘Conditional
Elimination’
1.P ⊃ Q2.Q3. ∴ P
Modus Ponens
Can be extended by Chain Argument
1. P 2. P ⊃ Q3. Q ⊃ R4. R ⊃ S5. ∴ S
to calculate the number of possible worlds
raise two to the power of the number of claims being evaluated, here there are two: P & Q
22 = 2 • 2 = 4
Proving Modus Ponens Valid
(P (P ⊃ Q) Q
1 True True True True
2 True True False False
3 False False True True
4 False False False False
Step One: Assign values to the simplest atomic claims, P & Q
P P Q Q
Proving Modus Ponens Valid
(P (P ⊃ Q) Q
1 True True True True True
2 True True False False False
3 False False True True True
4 False False True False False
Step Two: Determine the values of the next simplest or molecular claims.
Proving Modus Ponens Valid
(P (P ⊃ Q) Q
1 True ✔ True True True ✔ True
2 True True False False False
3 False False True True True
4 False False True False False
Step Three: Determine if there is a possible world where the premises are both true while the conclusion is false.
Proving Modus Ponens Valid
(P (P ⊃ Q) Q
1 True ✔ True True True ✔ True
2 True True False False False
3 False False True True True
4 False False True False False
There is no possible world where the premises are true while the conclusion is false.
So Modus Ponens Arguments are valid.
An Attendant Fallacy: Affirming the Consequent
There is a possible world where the premises are true while the conclusion is false.
So arguments which affirm the consequent are invalid—and so such arguments are fallacies
(Q (P ⊃ Q) P
1 True ✔ True True True ✔ True
2 False True False False True
3 True ✔ False True True ✘ False
4 False False True False False
A Common Form
Modus Tollens
modus tollens
1.If it is a mammal then it gives live birth.
2.It lays eggs.3. ∴ It’s not a mammal.
The Parts of a Modus Tollens Argument
Modus Tollens
It lays eggs.
If it is a mammal then it gives live birth.
∴ It’s not a mammal.
1.Conditional Premise2.Premise Denying the Consequent of the Conditional3.Concluding the Denial of the Antecedent of the Conditional
The Structure of a Modus Tollens Argument
Modus Ponens
1.If it is a mammal then it gives live birth.2.It lays eggs.3. ∴ It’s not a mammal.
A conditional premise.
The conclusion is the denial of the antecedent of the conditional premise.
A premise which denies the consequent of the conditional premise.
Modus Tollens
Also called ‘Denying the Consequent’
1.P ⊃ Q2.~Q3. ∴ ~P
Modus Tollens
Can be extended by Chain Argument
1. P ⊃ Q2. Q ⊃ R3. R ⊃ S4. ~S 5. ∴ ~P
to calculate the number of possible worlds
raise two to the power of the number of claims being evaluated, here there are two: P & Q
22 = 2 • 2 = 4
Proving Modus Tollens Valid
(~Q (P ⊃ Q) ~P
1 False True True False
2 True True False False
3 False False True True
4 True False False True
Step One: Assign values to the simplest atomic claims, P & Q, keeping track of negation.
~PP Q~Q
Proving Modus Tollens Valid
(~Q (P ⊃ Q) ~P
1 False True True True False
2 True True False False False
3 False False True True True
4 True False True False True
Step Two: Determine the values of the next simplest or molecular claims.
Proving Modus Tollens Valid
(~Q (P ⊃ Q) ~P
1 False True True True False
2 True True False False False
3 False False True True True
4 True ✔ False True False ✔ True
Step Three: Determine if there is a possible world where the premises are both true while the conclusion is false.
Proving Modus Tollens Valid
There is no possible world where the premises are true while the conclusion is false.
So Modus Tollens Arguments are valid.
(~Q (P ⊃ Q) ~P
1 False True True True False
2 True True False False False
3 False False True True True
4 True ✔ False True False ✔ True
An Attendant Fallacy: Denying the Antecedent
There is a possible world where the premises are true while the conclusion is false.
So arguments which affirm the consequent are invalid—and so such arguments are fallacies
(~P (P ⊃ Q) ~Q
1 False True True True False
2 False True False False True
3 True ✔ False True True ✘ False
4 True False True False True
Logical Complexity
Disjunction
A Logically Simple Truth
The coast is foggy.
1 True
2 False
Two logical possibilities
Given that we’ve filled
in the indices,
made the ceteris paribus
explicit, and defined key
terms.
Combining Logically Simple Truths
The coast is foggy. The coast is sunny.
1 True True
2 True False
3 False True
4 False False
Four states of affairs
(states) or possible worlds
Logical Conjunction
The coast is foggy. AND The coast is sunny.
1 True True True
2 True False False
3 False False True
4 False False False
Logical Conditional
IF The coast is foggy.THE
NThe coast is
sunny.
1 True True True
2 True False False
3 False True True
4 False True False
Logical Disjunction
The coast is foggy. OR The coast is sunny.
1 True ? True
2 True ? False
3 False ? True
4 False ? False
Logical Disjunction
The coast is foggy. OR The coast is sunny.
1 True ? True
2 True ? False
3 False ? True
4 False False False
Logical Disjunction
The coast is foggy. OR The coast is sunny.
1 True ? True
2 True True False
3 False True True
4 False False False
Logical Disjunction
The coast is foggy. OR The coast is sunny.
1 TrueTrue/False
True
2 True True False
3 False True True
4 False False False
The Ambiguity of ‘or’
Logical Disjunction
Exclusive ‘or’ Inclusive ‘or’
Either the Giants win the division or the A’s do (but
not both)
Either the Giants make the playoffs or the A’s do (or
both)
Either heads or tails (but not both)
Either by plane or by car (or both)
Latin: aut Latin: vel
Logical Disjunction
The coast is foggy. OR The coast is sunny.
1 True True True
2 True True False
3 False True True
4 False False False
Logic settles on an inclusive way
Logical disjunction, often expressed in English by ‘Either…or….’, is false when the both components are false, otherwise it is true. It is symbolized by ‘V’. It’s logical form is P V Q.
Logical Form of Disjunctions
Either P OR QP V Q
P V Q
1 TrueTrue
True
2 TrueTrue
False
3 FalseTrue
True
4 FalseFals
eFalse
to prove a disjunction true
Prove that one of the component claims is true.
to interpret a disjunction
specify if you are using it inclusively or exclusively.
Tautology
Eleven is a prime number oreleven is not a prime number.
Either Jacqui thinks black is more alluring than pink or she doesn’t.
The music is loud or the music is quiet.*
Jupiter is bigger than Mars or it is not bigger than Mars.
The Constitution of the United States was adopted on either September 17, 1787 or July 4, 1776.*
Romeo and Juliette is a tragedy or it is not a tragedy.*
New York either is or isn’t the largest city in the US.*
Hockey is better than basketball but it is not better than basketball.*
Putting Negation and Disjunction Together
Same-sex schools are optimal unless same-sex schools are less than optimal.
Drinking milk is healthy or unhealthy.*
The jellyfish has tentacles—or not!The child looks at the jellyfish or looks away from it*.
Which claim is not a disjunction?
Tautologies
The square is white
VThe square is not
white
1 True ? False
2 False ? True
The Logical Form of a Tautology: P V ~P
Given that disjunctions are false when all component claims are false, what is the truth value of this
disjunction?
Tautologies
The square is white
VThe square is not
white
1 TrueTrue
False
2 FalseTrue
True
Tautologies are true in all possible worlds.
The Logical Form of a Tautology: P V ~P
Tautology
P V ~P
1 TrueTrue
False
2 FalseTrue
True
The Logical Form of a Tautology: P V ~P
Tautologies are true in all possible worlds.
Tautology, a special form of disjunction in which a claim and its negation are joined—they are always true. The logical form of a tautology is P V ~P.
Tautology
T V ~T
1 True True False
2 False True True
The Logical Form of the Principle of Sufficient Reason
The Principle of Sufficient Reason covers all possible worlds.
For every claim, give a reason why it is true or not true.
Is drinking milk healthy for humans?
Controlling the Question
Has the Constitutional right to bear arms outlived its
usefulness?Are single-sex schools better for
education?What is the best method of
education?
What Constitutional rights should we keep?
What are the healthiest drinks for humans?
Controlling the QuestionOpen
QuestionsYes-or-no Questions
Is drinking milk healthy for humans?
Has the Constitutional right to bear arms
outlived its usefulness?
Are single-sex schools better for
education?
What is the best method of education?
What Constitutional rights should we
keep?
What are the healthiest drinks for humans?
Open Questions Yes-or-no Questions
What is the best method of education?
What Constitutional rights should we
keep?
What are the healthiest drinks for humans?
Are Topic or Theme Questions
A Topic Question: an open question. Such questions require disjunctive reasoning to treat the alternates.
Reasons supporting this alternate or
refuting the others.
Reasons supporting this alternate or
refuting the others.
Reasons supporting this alternate or
refuting the others.
the question
1st alternate:
Which alternate has the better
reasons?
Disjunctive Reasoning
2nd alternate:
Final alternate:
.
.
....
Reasons supporting electrolyte solutions or
refuting the others.
Reasons supporting milk or refuting the
others.
Reasons supporting water or refuting
the others.Water:
Which alternate has the better
reasons?
Disjunctive Reasoning
Milk:
Electrolyte Solutions
.
.
....
What are the healthiest drinks for humans?
Some Common Forms Involving Disjunctions
Validity
Disjunctive Argument
1. Either Kierkegaard can be a Christian or a philosopher.
2. He cannot be a philosopher.3. ∴ So he must be a Christian.
Simple Dilemma
Dilemma
1. If Johnny’s friendship is for pleasure then he is not a true friend.
2. If Johnny’s friendship is for utility then he is not a true friend.
3. Either Johnny’s friendship is for pleasure or utility.
4. ∴ Johnny’s friendship is not a true friendship.
1. If existence precedes essence then humanity is free.
2. If there is no God then we we alone can justify ourselves, without excuse.
3. Either existence precedes essence or there is not God.
4. ∴ Either humanity is free or is without any justifications or excuses but those they provide.
A Common Form
Disjunctive Argument
Disjunctive Argument
1.Either Kierkegaard can be a Christian or a philosopher.
2.He cannot be a philosopher.3. ∴ So he must be a Christian.
The Parts of a Disjunctive Argument
Disjunctive Argument
He cannot be a philosopher.
Either Kierkegaard can be a Christian or a philosopher.
∴ So he must be a Christian.
1.Disjunctive Premise.2.Premise Denying one of the disjuncts of the Disjunction.
3.Concluding the remaining Disjunct.
The Structure of a Disjunctive Argument
1.Either Kierkegaard can be a Christian or a philosopher.
2.He cannot be a philosopher.3. ∴ So he must be a Christian.
A disjunctive premise.
The conclusion is the remaining component claim of the disjunctive premise.
A premise which denies one of the component claims of the disjunctive premise.
Disjunctive Argument
Disjunctive Argument
More commonly called ‘Disjunctive Syllogism’ and also called ‘modus tollendo
ponens'
1. P V Q2. ~Q3. ∴ P
1. P V Q2. ~P3. ∴ Q
Can be run either way
Disjunctive Syllogism
Can be extended indefinitely
1. P V Q V R V S2. ~Q3. ~R4. ~S5. ∴ P
to calculate the number of possible worlds
raise two to the power of the number of claims being evaluated, here there are two: P & Q
22 = 2 • 2 = 4
Proving Disjunctive Argument Valid
~P (P V Q) Q
1 False True True True
2 False True False False
3 True False True True
4 True False False False
Step One: Assign values to the simplest atomic claims, P & Q, minding the negations.
~P P Q Q
Step Two: Determine the values of the next simplest or molecular claims.
~P (P V Q) Q
1 False True True True True
2 False True True False False
3 True False True True True
4 True False False False False
Proving Disjunctive Argument Valid
Step Three: Determine if there is a possible world where the premises are both true while the conclusion is false.
~P (P V Q) Q
1 False True True True True
2 False True True False False
3 True ✔ False True True ✔ True
4 True False False False False
Proving Disjunctive Argument Valid
There is no possible world where the premises are true while the conclusion is false.
So Disjunctive Arguments are valid.
Proving Disjunctive Argument Valid
~P (P V Q) Q
1 False True True True True
2 False True True False False
3 True ✔ False True True ✔ True
4 True False False False False
A Common Form
Simple Dilemma
Simple Dilemma
1.If Johnny’s friendship is for pleasure then he is not a true friend.
2.If Johnny’s friendship is for utility then he is not a true friend.
3.Either Johnny’s friendship is for pleasure or utility.
4. ∴ Johnny is not a true friend.
The Parts of a Simple Dilemma
Simple Dilemma
If Johnny’s friendship is for pleasure then he is not a true friend.
1. Disjunctive Premise.2. A Conditional Premise whose antecedent is one of the disjuncts of the
Disjunctive Premise and whose consequent is the same as the other Conditional Premise.
3. Another Conditional Premise whose antecedent is the other disjunct of the Disjunctive Premise and whose consequent is the same as the other Conditional Premise.
4. Concluding the Consequent of the Conditional Premises.
∴ Johnny is not a true friend.
If Johnny’s friendship is for utility then he is not a true friend.
Either Johnny’s friendship is for pleasure or utility.
The Structure of a Simple Dilemma
1.If Johnny’s friendship is for pleasure then he is not a true friend.
2.If Johnny’s friendship is for utility then he is not a true friend.3.Either Johnny’s friendship is for pleasure or utility.4. ∴ Johnny is not a true friend.
A disjunctive premise.
The conclusion is the consequent of the conditional premises.
Both Conditional Premises share a consequent.
Simple Dilemma
One antecedent is a component of the disjunction.
The other antecedent is the other component of the disjunction.
Simple Dilemma
Also called ‘Disjunctive Elimination’
1. P V Q2. P ⊃ R3. Q ⊃ R4. ∴ R
Simple Dilemma
Can be extended indefinitely
1. P V Q V R2. P ⊃ S3. Q ⊃ S4. R ⊃ S5. ∴ S
Destructive Dilemma
Simple Dilemma combined with Modus Tollens
1. ~R2. P ⊃ R3. Q ⊃ R4. ∴ ~P V Q
to calculate the number of possible worlds
raise two to the power of the number of claims being evaluated, here there are three: P, Q, & R
23 = 2 • 2 • 2 = 8
Proving Simple Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ R) R
1 True True True True True True True
2 True True TrueFals
eTrue
False
False
3 TrueFals
eTrue True
False
True True
4 TrueFals
eTrue
False
False
False
False
5 False
TrueFals
eTrue True True True
6 False
TrueFals
eFals
eTrue
False
False
7 False
False
False
TrueFals
eTrue True
8 False
False
False
False
False
False
False
Step One: Assign values to the simplest atomic claims, P, Q, & R
P PQ QR RR
Proving Simple Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ R) R
1 True True True True True True True True True True
2 True True True TrueFals
eFals
eTrue
False
False
False
3 True TrueFals
eTrue True True
False
True True True
4 True TrueFals
eTrue
False
False
False
TrueFals
eFals
e
5 False
True TrueFals
eTrue True True True True True
6 False
True TrueFals
eTrue
False
TrueFals
eFals
eFals
e
7 False
False
False
False
True TrueFals
eTrue True True
8 False
False
False
False
TrueFals
eFals
eTrue
False
False
Step Two: Determine the values of the next simplest or molecular claims.
Proving Simple Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ R) R
1 True True True✔True True True✔True True True✔True
2 True True True TrueFals
eFals
eTrue
False
False
False
3 True TrueFals
e ✔True True True✔
False
True True✔True
4 True TrueFals
eTrue
False
False
False
TrueFals
eFals
e
5 False
True True✔Fals
eTrue True✔True True True✔True
6 False
True TrueFals
eTrue
False
TrueFals
eFals
eFals
e
7 False
False
False
False
True TrueFals
eTrue True True
8 False
False
False
False
TrueFals
eFals
eTrue
False
False
Step Three: Determine if there is a possible world where the premises are both true while the conclusion is false.
There is no possible world where the premises are true while the conclusion is false.
So Simple Dilemmas are valid.
Proving Simple Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ R) R
1 True True True✔True True True✔True True True✔True
2 True True True TrueFals
eFals
eTrue
False
False
False
3 True TrueFals
e ✔True True True✔
False
True True✔True
4 True TrueFals
eTrue
False
False
False
TrueFals
eFals
e
5 False
True True✔Fals
eTrue True✔True True True✔True
6 False
True TrueFals
eTrue
False
TrueFals
eFals
eFals
e
7 False
False
False
False
True TrueFals
eTrue True True
8 False
False
False
False
TrueFals
eFals
eTrue
False
False
A Common Form
Dilemma
Dilemma
1.If existence precedes essence then humanity is free.
2.If there is no God then we we alone can justify ourselves, without excuse.
3.Either existence precedes essence or there is no God.
4. ∴ Either humanity is free or is without any justifications or excuses but those they provide.
If there is no God then we we alone can justify ourselves, without excuse.
The Parts of a Dilemma
Dilemma
If existence precedes essence then humanity is free.
1. Disjunctive Premise.2. A Conditional Premise whose antecedent is one of the disjuncts of the
Disjunctive Premise.3. Another Conditional Premise whose antecedent is the other disjunct of the
Disjunctive Premise.4. Concluding a Disjunction of the Consequents of the Conditional Premises.
∴ Either humanity is free or is without any justifications or excuses but those they provide.
Either existence precedes essence or there is no God.
The Structure of a Dilemma
1. If existence precedes essence then humanity is free.2. If there is no God then we we alone can justify ourselves, without
excuse.3. Either existence precedes essence or there is no God.4. ∴ Either humanity is free or is without any justifications or
excuses but those they provide.
A disjunctive premise.
The conclusion is a disjunction of the consequents of the conditional premises.
Dilemma
One antecedent is a component of the disjunction.
The other antecedent is the other component of the disjunction.
Dilemma
Also called ‘Constructive Dilemma’
1. P V Q2. P ⊃ R3. Q ⊃ S4. ∴ R V S
Simple Dilemma
Can be extended indefinitely
1. P V Q V R2. P ⊃ S3. Q ⊃ T4. R ⊃ U5. ∴ S V T V U
Destructive Dilemma
Dilemma combined with Modus Tollens
1. ~R V ~S2. P ⊃ R3. Q ⊃ S4. ∴ ~P V ~Q
Destructive Dilemma
Dilemma combined with Modus Tollens and a Tautology
1. ~R V ~R2. P ⊃ R3. Q ⊃ R4. ∴ ~P V ~Q
to calculate the number of possible worlds
raise two to the power of the number of claims being evaluated, here there are three: P, Q, & R
24 = 2 • 2 • 2 • 2 = 16
Proving Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ S) R V S
1 True True True True True True True True
2 True True True True True False True False
3 True True True False True True False True
4 True True True False True False False False
5 True False True True False True True True
6 True False True True False False True False
7 True False True False False True False True
8 True False True False False False False False
9 False True False True True True True True
10 False True False True True False True False
11 False True False False True True False True
12 False True False False True False False False
13 False False False True False True True True
14 False False False True False False True False
15 False False False False False True False True
16 False False False False False False False False
Step One: Assign values to the simplest atomic claims, P, Q, R, & S
RP Q SP Q R S
Proving Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ S) R V S
1 True True True True True True True True True True True True
2 True True True True True True True False False True True False
3 True True True True False False True True True False True True
4 True True True True False False True False False False False False
5 True True False True True True False True True True True True
6 True True False True True True False True False True True False
7 True True False True False False False True True False True True
8 True True False True False False False True False False False False
9 False True True False True True True True True True True True
10 False True True False True True True False False True True False
11 False True True False True False True True True False True True
12 False True True False True False True False False False False False
13 False False False False True True False True True True True True
14 False False False False True True False True False True True False
15 False False False False True False False True True False True True
16 False False False False True False False True False False False False
Step Two: Determine the values of the next simplest or molecular claims.
Proving Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ S) R V S
1 True True True ✔ True True True ✔ True True True ✔ True True True
2 True True True True True True True False False True True False
3 True True True True False False True True True False True True
4 True True True True False False True False False False False False
5 True True False ✔ True True True ✔ False True True ✔ True True True
6 True True False ✔ True True True ✔ False True False ✔ True True False
7 True True False True False False False True True False True True
8 True True False True False False False True False False False False
9 False True True ✔ False True True ✔ True True True ✔ True True True
10 False True True False True True True False False True True False
11 False True True ✔ False True False ✔ True True True ✔ False True True
12 False True True False True False True False False False False False
13 False False False False True True False True True True True True
14 False False False False True True False True False True True False
15 False False False False True False False True True False True True
16 False False False False True False False True False False False False
Step Three: Determine if there is a possible world where the premises are both true while the conclusion is false.
Proving Dilemma Valid
(P V Q) (P ⊃ R) (Q ⊃ S) R V S
1 True True True ✔ True True True ✔ True True True ✔ True True True
2 True True True True True True True False False True True False
3 True True True True False False True True True False True True
4 True True True True False False True False False False False False
5 True True False ✔ True True True ✔ False True True ✔ True True True
6 True True False ✔ True True True ✔ False True False ✔ True True False
7 True True False True False False False True True False True True
8 True True False True False False False True False False False False
9 False True True ✔ False True True ✔ True True True ✔ True True True
10 False True True False True True True False False True True False
11 False True True ✔ False True False ✔ True True True ✔ False True True
12 False True True False True False True False False False False False
13 False False False False True True False True True True True True
14 False False False False True True False True False True True False
15 False False False False True False False True True False True True
16 False False False False True False False True False False False False
There is no possible world where the premises are true while the conclusion is false.
So Dilemmas are valid.
A Relevant Fallacy
Fallacies
False Dilemma, to provide a non exhaustive disjunction as a premise—it is a fallacy.
to avoid false dilemma
Provide an exhaustive list of the possible answers to the topic question, listing explicitly those you may not wish to treat.
Ethics
Assignment
How do you come to an ethical decision?
What is hypocrisy?
What does it mean to be ethical?