Transcript

Chapter 5

Methods of Proof for Boolean

Logic

Truth tables give us powerful techniques for investigating the logic of the

Boolean operators. But they are by no means the end of the story. Truth

tables are fine for showing the validity of simple arguments that depend only

on truth-functional connectives, but the method has two very significant lim-

itations.

First, truth tables get extremely large as the number of atomic sentenceslimitations of truth

table methods goes up. An argument involving seven atomic sentences is hardly unusual, but

testing it for validity would call for a truth table with 27 = 128 rows. Testing

an argument with 14 atomic sentences, just twice as many, would take a table

containing over 16 thousand rows. You could probably get a Ph.D. in logic for

building a truth table that size. This exponential growth severely limits the

practical value of the truth table method.

The second limitation is, surprisingly enough, even more significant. Truth

table methods can’t be easily extended to reasoning whose validity depends

on more than just truth-functional connectives. As you might guess from the

artificiality of the arguments looked at in the previous chapter, this rules out

most kinds of reasoning you’ll encounter in everyday life. Ordinary reasoning

relies heavily on the logic of the Boolean connectives, make no mistake about

that. But it also relies on the logic of other kinds of expressions. Since the

truth table method detects only tautological consequence, we need a method

of applying Boolean logic that can work along with other valid principles of

reasoning.

Methods of proof, both formal and informal, give us the required exten-

sibility. In this chapter we will discuss legitimate patterns of inference that

arise when we introduce the Boolean connectives into a language, and show

how to apply the patterns in informal proofs. In Chapter 6, we’ll extend our

formal system with corresponding rules. The key advantage of proof methods

over truth tables is that we’ll be able to use them even when the validity of

our proof depends on more than just the Boolean operators.

The Boolean connectives give rise to many valid patterns of inference.

Some of these are extremely simple, like the entailment from the sentence

P ∧ Q to P. These we will refer to as valid inference steps, and will discuss

128

Valid inference steps / 129

them briefly in the first section. Much more interesting are two new methods

of proof that are allowed by the new expressions: proof by cases and proof by

contradiction. We will discuss these later, one at a time.

Section 5.1

Valid inference steps

Here’s an important rule of thumb: In an informal proof, it is always legiti-

mate to move from a sentence P to another sentence Q if both you and your

“audience” (the person or people you’re trying to convince) already know important rule of thumb

that Q is a logical consequence of P. The main exception to this rule is when

you give informal proofs to your logic instructor: presumably, your instructor

knows the assigned argument is valid, so in these circumstances, you have to

pretend you’re addressing the proof to someone who doesn’t already know

that. What you’re really doing is convincing your instructor that you see that

the argument is valid and that you could prove it to someone who did not.

The reason we start with this rule of thumb is that you’ve already learned

several well-known logical equivalences that you should feel free to use when

giving informal proofs. For example, you can freely use double negation or

idempotence if the need arises in a proof. Thus a chain of equivalences of the

sort we gave on page 120 is a legitimate component of an informal proof. Of

course, if you are asked to prove one of the named equivalences, say one of

the distribution or DeMorgan laws, then you shouldn’t presuppose it in your

proof. You’ll have to figure out a way to prove it to someone who doesn’t

already know that it is valid.

A special case of this rule of thumb is the following: If you already know

that a sentence Q is a logical truth, then you may assert Q at any point in

your proof. We already saw this principle at work in Chapter 2, when we

discussed the reflexivity of identity, the principle that allowed us to assert a

sentence of the form a = a at any point in a proof. It also allows us to assert

other simple logical truths, like excluded middle (P ∨ ¬P), at any point in a

proof. Of course, the logical truths have to be simple enough that you can be

sure your audience will recognize them.

There are three simple inference steps that we will mention here that don’t

involve logical equivalences or logical truths, but that are clearly supported

by the meanings of ∧ and ∨. First, suppose we have managed to prove a

conjunction, say P ∧ Q, in the course of our proof. The individual conjuncts

P and Q are clearly consequences of this conjunction, because there is no way

for the conjunction to be true without each conjunct being true. Thus, we

Section 5.1

130 / Methods of Proof for Boolean Logic

are justified in asserting either. More generally, we are justified in inferring,

from a conjunction of any number of sentences, any one of its conjuncts. Thisconjunction

elimination

(simplification)inference pattern is sometimes called conjunction elimination or simplification,

when it is presented in the context of a formal system of deduction. When it

is used in informal proofs, however, it usually goes by without comment, since

it is so obvious.

Only slightly more interesting is the converse. Given the meaning of ∧, it

is clear that P ∧ Q is a logical consequence of the pair of sentences P and Q:

there is no way the latter could be true without former also being true. Thus

if we have managed to prove P and to prove Q from the same premises, then

we are entitled to infer the conjunction P ∧ Q. More generally, if we want toconjunction

introduction prove a conjunction of a bunch of sentences, we may do so by proving each

conjunct separately. In a formal system of deduction, steps of this sort are

sometimes called conjunction introduction or just conjunction. Once again,

in real life reasoning, these steps are too simple to warrant mention. In our

informal proofs, we will seldom point them out explicitly.

Finally, let us look at one valid inference pattern involving ∨. It is a simple

step, but one that strikes students as peculiar. Suppose that you have proven

Cube(b). Then you can conclude Cube(a) ∨ Cube(b) ∨ Cube(c), if you shoulddisjunction

introduction want to for some reason, since the latter is a consequence of the former.

More generally, if you have proven some sentence P then you can infer any

disjunction that has P as one of its disjuncts. After all, if P is true, so is any

such disjunction.

What strikes newcomers to logic as peculiar about such a step is that using

it amounts to throwing away information. Why in the world would you want

to conclude P ∨ Q when you already know the more informative claim P? But

as we will see, this step is actually quite useful when combined with some

of the methods of proof to be discussed later. Still, in mathematical proofs,

it generally goes by unnoticed. In formal systems, it is dubbed disjunction

introduction, or (rather unfortunately) addition.

Matters of style

Informal proofs serve two purposes. On the one hand, they are a method of

discovery; they allow us to extract new information from information already

obtained. On the other hand, they are a method of communication; they allow

us to convey our discoveries to others. As with all forms of communication,

this can be done well or done poorly.

When we learn to write, we learn certain basic rules of punctuation, capi-

talization, paragraph structure and so forth. But beyond the basic rules, there

are also matters of style. Different writers have different styles. And it is a

Chapter 5

Valid inference steps / 131

good thing, since we would get pretty tired of reading if everyone wrote with

the very same style. So too in giving proofs. If you go on to study mathemat-

ics, you will read lots of proofs, and you will find that every writer has his or

her own style. You will even develop a style of your own.

Every step in a “good” proof, besides being correct, should have two prop-

erties. It should be easily understood and significant. By “easily understood”

we mean that other people should be able to follow the step without undue

difficulty: they should be able to see that the step is valid without having to

engage in a piece of complex reasoning of their own. By “significant” we mean

that the step should be informative, not a waste of the reader’s time.

These two criteria pull in opposite directions. Typically, the more signif-

icant the step, the harder it is to follow. Good style requires a reasonable

balance between the two. And that in turn requires some sense of who your knowing your audience

audience is. For example, if you and your audience have been working with

logic for a while, you will recognize a number of equivalences that you will

want to use without further proof. But if you or your audience are beginners,

the same inference may require several steps.

Remember

1. In giving an informal proof from some premises, if Q is already

known to be a logical consequence of sentences P1, . . . ,Pn and each of

P1, . . . ,Pn has been proven from the premises, then you may assert Q

in your proof.

2. Each step in an informal proof should be significant but easily under-

stood.

3. Whether a step is significant or easily understood depends on the

audience to whom it is addressed.

4. The following are valid patterns of inference that generally go unmen-

tioned in informal proofs:

◦ From P ∧ Q, infer P.

◦ From P and Q, infer P ∧ Q.

◦ From P, infer P ∨ Q.

Section 5.1

132 / Methods of Proof for Boolean Logic

Exercises

In the following exercises we list a number of patterns of inference, only some of which are valid. For

each pattern, determine whether it is valid. If it is, explain why it is valid, appealing to the truth tables

for the connectives involved. If it is not, give a specific example of how the step could be used to get from

true premises to a false conclusion.

5.1.

From P ∨ Q and ¬P, infer Q. 5.2.

From P ∨ Q and Q, infer ¬P.

5.3.

From ¬(P ∨ Q), infer ¬P. 5.4.

From ¬(P ∧ Q) and P, infer ¬Q.

5.5.

From ¬(P ∧ Q), infer ¬P. 5.6.⋆

From P ∧ Q and ¬P, infer R.

Section 5.2

Proof by cases

The simple forms of inference discussed in the last section are all instances of

the principle that you can use already established cases of logical consequence

in informal proofs. But the Boolean connectives also give rise to two entirely

new methods of proof, methods that are explicitly applied in all types of

rigorous reasoning. The first of these is the method of proof by cases. In our

formal system F , this method will be called disjunction elimination, but don’t

be misled by the ordinary sounding name: it is far more significant than, say,

disjunction introduction or conjunction elimination.

We begin by illustrating proof by cases with a well-known piece of math-

ematical reasoning. The reasoning proves that there are irrational numbers b

and c such that bc is rational. First, let’s review what this means. A number

is said to be rational if it can be expressed as a fraction n/m, for integers

n and m. If it can’t be so expressed, then it is irrational. Thus 2 is rational

(2 = 2/1), but√

2 is irrational. (We will prove this latter fact in the next sec-

tion, to illustrate proof by contradiction; for now, just take it as a well-known

truth.) Here now is our proof:

Proof: To show that there are irrational numbers b and c such that

bc is rational, we will consider the number√

2√2. We note that this

number is either rational or irrational.

Chapter 5

Proof by cases / 133

If√

2√2

is rational, then we have found our b and c; namely, we take

b = c =√

2.

Suppose, on the other hand, that√

2√2

is irrational. Then we take

b =√

2√2

and c =√

2 and compute bc:

bc = (√

2√2)√2

=√

2(√2·√2)

=√

22

= 2

Thus, we see that in this case, too, bc is rational.

Consequently, whether√

2√2

is rational or irrational, we know that

there are irrational numbers b and c such that bc is rational.

What interests us here is not the result itself but the general structure of

the argument. We begin with a desired goal that we want to prove, say S, and

a disjunction we already know, say P ∨ Q. We then show two things: that S proof by cases

follows if we assume that P is the case, and that S follows if we assume that

Q is the case. Since we know that one of these must hold, we then conclude

that S must be the case. This is the pattern of reasoning known as proof by

cases.

In proof by cases, we aren’t limited to breaking into just two cases, as we

did in the example. If at any stage in a proof we have a disjunction containing

n disjuncts, say P1∨ . . . ∨ Pn, then we can break into n cases. In the first we

assume P1, in the second P2, and so forth for each disjunct. If we are able to

prove our desired result S in each of these cases, we are justified in concluding

that S holds.

Let’s look at an even simpler example of proof by cases. Suppose we want

to prove that Small(c) is a logical consequence of

(Cube(c) ∧ Small(c)) ∨ (Tet(c) ∧ Small(c))

This is pretty obvious, but the proof involves breaking into cases, as you will

notice if you think carefully about how you recognize this. For the record,

here is how we would write out the proof.

Proof: We are given

(Cube(c) ∧ Small(c)) ∨ (Tet(c) ∧ Small(c))

as a premise. We will break into two cases, corresponding to the two

disjuncts. First, assume that Cube(c) ∧ Small(c) holds. But then (by

Section 5.2

134 / Methods of Proof for Boolean Logic

conjunction elimination, which we really shouldn’t even mention) we

have Small(c). But likewise, if we assume Tet(c) ∧ Small(c), then it

follows that Small(c). So, in either case, we have Small(c), as desired.

Our next example shows how the odd step of disjunction introduction

(from P infer P ∨ Q) can be used fruitfully with proof by cases. Suppose we

know that either Max is home and Carl is happy, or Claire is home and Scruffy

is happy, i.e.,

(Home(max) ∧ Happy(carl)) ∨ (Home(claire) ∧ Happy(scruffy))

We want to prove that either Carl or Scruffy is happy, that is,

Happy(carl) ∨ Happy(scruffy)

A rather pedantic, step-by-step proof would look like this:

Proof: Assume the disjunction:

(Home(max) ∧ Happy(carl)) ∨ (Home(claire) ∧ Happy(scruffy))

Then either:

Home(max) ∧ Happy(carl)

or:

Home(claire) ∧ Happy(scruffy).

If the first alternative holds, then Happy(carl), and so we have

Happy(carl) ∨ Happy(scruffy)

by disjunction introduction. Similarly, if the second alternative holds,

we have Happy(scruffy), and so

Happy(carl) ∨ Happy(scruffy)

So, in either case, we have our desired conclusion. Thus our conclu-

sion follows by proof by cases.

Arguing by cases is extremely useful in everyday reasoning. For example,

one of the authors (call him J) and his wife recently realized that their parking

meter had expired several hours earlier. J argued in the following way that

there was no point in rushing back to the car (logicians argue this way; don’t

marry one):

Chapter 5

Proof by cases / 135

Proof: At this point, either we’ve already gotten a ticket or we

haven’t. If we’ve gotten a ticket, we won’t get another one in the

time it takes us to get to the car, so rushing would serve no purpose.

If we haven’t gotten a ticket in the past several hours, it is extremely

unlikely that we will get one in the next few minutes, so again,

rushing would be pointless. In either event, there’s no need to rush.

J’s wife responded with the following counterargument (showing that many

years of marriage to a logician has an impact):

Proof: Either we are going to get a ticket in the next few minutes or

we aren’t. If we are, then rushing might prevent it, which would be

a good thing. If we aren’t, then it will still be good exercise and will

also show our respect for the law, both of which are good things. So

in either event, rushing back to the car is a good thing to do.

J’s wife won the argument.

The validity of proof by cases cannot be demonstrated by the simple truth

table method introduced in Chapter 4. The reason is that we infer the con-

clusion S from the fact that S is provable from each of the disjuncts P and

Q. It relies on the principle that if S is a logical consequence of P, and also a

logical consequence of Q, then it is a logical consequence of P ∨ Q. This holds

because any circumstance that makes P ∨ Q true must make at least one of P

or Q true, and hence S as well, by the fact that S is a consequence of both.

Remember

Proof by cases: To prove S from P1 ∨ . . . ∨ Pn using this method, prove

S from each of P1, . . . , Pn.

Exercises

The next two exercises present valid arguments. Turn in informal proofs of the arguments’ validity. Your

proofs should be phrased in complete, well-formed English sentences, making use of first-order sentences

as convenient, much in the style we have used above. Whenever you use proof by cases, say so. You don’t

have to be explicit about the use of simple proof steps like conjunction elimination. By the way, there is

typically more than one way to prove a given result.

Section 5.2

136 / Methods of Proof for Boolean Logic

5.7.

Home(max) ∨ Home(claire)

¬Home(max) ∨ Happy(carl)

¬Home(claire) ∨ Happy(carl)

Happy(carl)

5.8.

LeftOf(a, b) ∨ RightOf(a, b)

BackOf(a, b) ∨ ¬LeftOf(a, b)

FrontOf(b, a) ∨ ¬RightOf(a, b)

SameCol(c, a) ∧ SameRow(c, b)

BackOf(a, b)

5.9ö|.

Assume the same four premises as in Exercise 5.8. Is LeftOf(b, c) a logical consequence of

these premises? If so, turn in an informal proof of the argument’s validity. If not, submit a

counterexample world.

5.10.

Suppose Max’s favorite basketball team is the Chicago Bulls and favorite football team is the

Denver Broncos. Max’s father John is returning from Indianapolis to San Francisco on United

Airlines, and promises that he will buy Max a souvenir from one of his favorite teams on the

way. Explain John’s reasoning, appealing to the annoying fact that all United flights between

Indianapolis and San Francisco stop in either Denver or Chicago. Make explicit the role proof

by cases plays in this reasoning.

5.11.

Suppose the police are investigating a burglary and discover the following facts. All the doors

to the house were bolted from the inside and show no sign of forced entry. In fact, the only

possible ways in and out of the house were a small bathroom window on the first floor that

was left open and an unlocked bedroom window on the second floor. On the basis of this, the

detectives rule out a well-known burglar, Julius, who weighs two hundred and fifty pounds and

is arthritic. Explain their reasoning.

5.12.

In our proof that there are irrational numbers b and c where bc is rational, one of our steps

was to assert that√

2√2

is either rational or irrational. What justifies the introduction of this

claim into our proof?

5.13.

Describe an everyday example of reasoning by cases that you have performed in the last few

days.

5.14.⋆

Give an informal proof that if S is a tautological consequence of P and a tautological conse-

quence of Q, then S is a tautological consequence of P ∨ Q. Remember that the joint truth

table for P ∨ Q and S may have more rows than either the joint truth table for P and S, or the

joint truth table for Q and S. [Hint: Assume you are looking at a single row of the joint truth

table for P ∨ Q and S in which P ∨ Q is true. Break into cases based on whether P is true or Q

is true and prove that S must be true in either case.]

Chapter 5

Indirect proof: proof by contradiction / 137

Section 5.3

Indirect proof: proof by contradiction

One of the most important methods of proof is known as proof by contradic-

tion. It is also called indirect proof or reductio ad absurdum. Its counterpart

in F is called negation introduction.

The basic idea is this. Suppose you want to prove a negative sentence, say

¬S, from some premises, say P1, . . . ,Pn. One way to do this is by temporarily indirect proof or proof

by contradictionassuming S and showing that a contradiction follows from this assumption. If

you can show this, then you are entitled to conclude that ¬S is a logical conse-

quence of the original premises. Why? Because your proof of the contradiction

shows that S, P1, . . . ,Pn cannot all be true simultaneously. (If they were, the

contradiction would have to be true, and it can’t be.) Hence if P1, . . . ,Pn are

true in any set of circumstances, then S must be false in those circumstances.

Which is to say, if P1, . . . ,Pn are all true, then ¬S must be true as well.

Let’s look at a simple indirect proof. Assume Cube(c) ∨ Dodec(c) and

Tet(b). Let us prove ¬(b = c).

Proof: In order to prove ¬(b = c), we assume b = c and attempt

to get a contradiction. From our first premise we know that either

Cube(c) or Dodec(c). If the first is the case, then we conclude Cube(b)

by the indiscernibility of identicals, which contradicts Tet(b). But

similarly, if the second is the case, we get Dodec(b) which contra-

dicts Tet(b). So neither case is possible, and we have a contradiction.

Thus our initial assumption that b = c must be wrong. So proof by

contradiction gives us our desired conclusion, ¬(b = c). (Notice that

this argument also uses the method of proof by cases.)

Let us now give a more interesting and famous example of this method of

proof. The Greeks were shocked to discover that the square root of 2 could

not be expressed as a fraction, or, as we would put it, is irrational. The proof

of this fact proceeds via contradiction. Before we go through the proof, let’s

review some simple numerical facts that were well known to the Greeks. The

first is that any rational number can be expressed as a fraction p/q where at

least one of p and q is odd. (If not, keep dividing both the numerator and

denominator by 2 until one of them is odd.) The other fact follows from the

observation that when you square an odd number, you always get an odd

number. So if n2 is an even number, then so is n. And from this, we see that

if n2 is even, it must be divisible by 4.

Now we’re ready for the proof that√

2 is irrational.

Section 5.3

138 / Methods of Proof for Boolean Logic

Proof: With an eye toward getting a contradiction, we will assume

that√

2 is rational. Thus, on this assumption,√

2 can be expressed

in the form p/q, where at least one of p and q is odd. Since p/q =√

2

we can square both sides to get:

p2

q2= 2

Multiplying both sides by q2, we get p2 = 2q2. But this shows that

p2 is an even number. As we noted before, this allows us to conclude

that p is even and that p2 is divisible by 4. Looking again at the

equation p2 = 2q2, we see that if p2 is divisible by 4, then 2q2 is

divisible by 4 and hence q2 must be divisible by 2. In which case, q is

even as well. So both p and q are even, contradicting the fact that at

least one of them is odd. Thus, our assumption that√

2 is rational

led us to a contradiction, and so we conclude that it is irrational.

In both of these examples, we used the method of indirect proof to prove a

sentence that begins with a negation. (Remember, “irrational” simply means

not rational.) You can also use this method to prove a sentence S that does not

begin with a negation. In this case, you would begin by assuming ¬S, obtain

a contradiction, and then conclude that ¬¬S is the case, which of course is

equivalent to S.

In order to apply the method of proof by contradiction, it is important

that you understand what a contradiction is, since that is what you need

to prove from your temporary assumption. Intuitively, a contradiction is anycontradiction

claim that cannot possibly be true, or any set of claims which cannot all

be true simultaneously. Examples are a sentence Q and its negation ¬Q, a

pair of inconsistent claims like Cube(c) and Tet(c) or x < y and y < x, or a

single sentence of the form a ̸= a. We can take the notion of a contradictory

or inconsistent set of sentences to be any set of sentences that could not all

be true in any single situation.

The symbol ⊥ is often used as a short-hand way of saying that a contra-contradiction

symbol (⊥) diction has been obtained. Different people read ⊥ as “contradiction,” “the

absurd,” and “the false,” but what it means is that a conclusion has been

reached which is logically impossible, or that several conclusions have been

derived which, taken together, are impossible.

Notice that a sentence S is a logical impossibility if and only if its negation

¬S is logically necessary. This means that any method we have of demonstrat-

ing that a sentence is logically necessary also demonstrates that its negation

is logically impossible, that is, a contradiction. For example, if a truth table

shows that ¬S is a tautology, then we know that S is a contradiction.

Chapter 5

Indirect proof: proof by contradiction / 139

Similarly, the truth table method gives us a way of showing that a col-

lection of sentences are mutually contradictory. Construct a joint truth table

for P1, . . . ,Pn. These sentences are tt-contradictory if every row has an F as- tt-contradictory

signed to at least one of the sentences. If the sentences are tt-contradictory,

we know they cannot all be true at once, simply in virtue of the meanings

of the truth functional connectives out of which they are built. We have al-

ready mentioned one such example: any pair of sentences, one of which is the

negation of the other.

The method of proof by contradiction, like proof by cases, is often encoun-

tered in everyday reasoning, though the derived contradiction is sometimes

left implicit. People will often assume a claim for the sake of argument and

then show that the assumption leads to something else that is known to be

false. They then conclude the negation of the original claim. This sort of rea-

soning is in fact an indirect proof: the inconsistency becomes explicit if we

add the known fact to our set of premises.

Let’s look at an example of this kind of reasoning. Imagine a defense

attorney presenting the following summary to the jury:

The prosecution claims that my client killed the owner of the KitKat

Club. Assume that they are correct. You’ve heard their own experts

testify that the murder took place at 5:15 in the afternoon. We also

know the defendant was still at work at City Hall at 4:45, according

to the testimony of five co-workers. It follows that my client had to

get from City Hall to the KitKat Club in 30 minutes or less. But

to make that trip takes 35 minutes under the best of circumstances,

and police records show that there was a massive traffic jam the day

of the murder. I submit that my client is innocent.

Clearly, reasoning like this is used all the time: whenever we assume some-

thing and then rule out the assumption on the basis of its consequences.

Sometimes these consequences are not contradictions, or even things that we

know to be false, but rather future consequences that we consider unaccept-

able. You might for example assume that you will go to Hawaii for spring

break, calculate the impact on your finances and ability to finish the term

papers coming due, and reluctantly conclude that you can’t make the trip.

When you reason like this, you are using the method of indirect proof.

Remember

Proof by contradiction: To prove ¬S using this method, assume S and

prove a contradiction ⊥.

Section 5.3

140 / Methods of Proof for Boolean Logic

Exercises

In the following exercises, decide whether the displayed argument is valid. If it is, turn in an infor-

mal proof, phrased in complete, well-formed English sentences, making use of first-order sentences as

convenient. Whenever you use proof by cases or proof by contradiction, say so. You don’t have to be

explicit about the use of simple proof steps like conjunction elimination. If the argument is invalid, con-

struct a counterexample world in Tarski’s World. (Argument 5.16 is valid, and so will not require a

counterexample.)

5.15ö|.

b is a tetrahedron.

c is a cube.

Either c is larger than b or else they

are identical.

b is smaller than c.

5.16.

Max or Claire is at home but either

Scruffy or Carl is unhappy.

Either Max is not home or Carl is

happy.

Either Claire is not home or Scruffy is

unhappy.

Scruffy is unhappy.

5.17ö|.

Cube(a) ∨ Tet(a) ∨ Large(a)

¬Cube(a) ∨ a = b ∨ Large(a)

¬Large(a) ∨ a = c

¬(c = c ∧ Tet(a))

a = b ∨ a = c

5.18ö|.

Cube(a) ∨ Tet(a) ∨ Large(a)

¬Cube(a) ∨ a = b ∨ Large(a)

¬Large(a) ∨ a = c

¬(c = c ∧ Tet(a))

¬(Large(a) ∨ Tet(a))

5.19.

Consider the following sentences.

1. Folly was Claire’s pet at 2 pm or at 2:05 pm.

2. Folly was Max’s pet at 2 pm.

3. Folly was Claire’s pet at 2:05 pm.

Does (3) follow from (1) and (2)? Does (2) follow from (1) and (3)? Does (1) follow from (2)

and (3)? In each case, give either a proof of consequence, or describe a situation that makes the

premises true and the conclusion false. You may assume that Folly can only be one person’s

pet at any given time.

5.20.

Suppose it is Friday night and you are going out with your boyfriend. He wants to see a romantic

comedy, while you want to see the latest Wes Craven slasher movie. He points out that if he

watches the Wes Craven movie, he will not be able to sleep because he can’t stand the sight of

blood, and he has to take the MCAT test tomorrow. If he does not do well on the MCAT, he

won’t get into medical school. Analyze your boyfriend’s argument, pointing out where indirect

proof is being used. How would you rebut his argument?

Chapter 5

Arguments with inconsistent premises / 141

5.21.

Describe an everyday example of an indirect proof that you have used in the last few days.

5.22.⋆

Prove that indirect proof is a tautologically valid method of proof. That is, show that if

P1, . . . ,Pn, S is tt-contradictory, then ¬S is a tautological consequence of P1, . . . ,Pn.

In the next three exercises we ask you to prove simple facts about the natural numbers. We do not expect

you to phrase the proofs in fol. You will have to appeal to basic facts of arithmetic plus the definitions

of even and odd number. This is OK, but make these appeals explicit. Also make explicit any use of proof

by contradiction.

5.23.⋆

Assume that n2 is

odd. Prove that n is

odd.

5.24.⋆

Assume that n + m

is odd. Prove that

n×m is even.

5.25.⋆

Assume that n2 is

divisible by 3. Prove

that n2 is divisible

by 9.

5.26.⋆⋆

A good way to make sure you understand a proof is to try to generalize it. Prove that√

3 is

irrational. [Hint: You will need to figure out some facts about divisibility by 3 that parallel the

facts we used about even and odd, for example, the fact expressed in Exercise 5.25.] Can you

generalize these two results?

Section 5.4

Arguments with inconsistent premises

What follows from an inconsistent set of premises? If you look back at our

definition of logical consequence, you will see that every sentence is a conse-

quence of such a set. After all, if the premises are contradictory, then there

are no circumstances in which they are all true. Thus, there are no circum-

stances in which the premises are true and the conclusion is false. Which is

to say, in any situation in which the premises are all true (there aren’t any

of these!), the conclusion will be true as well. Hence any argument with an always valid

inconsistent set of premises is trivially valid. In particular, if one can establish

a contradiction ⊥ on the basis of the premises, then one is entitled to assert

any sentence at all.

This often strikes students as a very odd method of reasoning, and for very

good reason. For recall the distinction between a valid argument and a sound

one. A sound argument is a valid argument with true premises. Even though

any argument with an inconsistent set of premises is valid, no such argument

is sound, since there is no way the premises of the argument can all be true.

For this reason, an argument with an inconsistent set of premises is not worth

Section 5.4

142 / Methods of Proof for Boolean Logic

much on its own. After all, the reason we are interested in logical consequencenever sound

is because of its relation to truth. If the premises can’t possibly be true, then

even knowing that the argument is valid gives us no clue as to the truth or

falsity of the conclusion. An unsound argument gives no more support for its

conclusion than an invalid one.

In general, methods of proof don’t allow us to show that an argument

is unsound. After all, the truth or falsity of the premises is not a matter of

logic, but of how the world happens to be. But in the case of arguments with

inconsistent premises, our methods of proof do give us a way to show that at

least one of the premises is false (though we might not know which one), and

hence that the argument is unsound. To do this, we prove that the premises

are inconsistent by deriving a contradiction.

Suppose, for example, you are given a proof that the following argument

is valid:

Home(max) ∨ Home(claire)

¬Home(max)

¬Home(claire)

Home(max) ∧ Happy(carl)

While it is true that this conclusion is a consequence of the premises, your

reaction should not be to believe the conclusion. Indeed, using proof by cases

we can show that the premises are inconsistent, and hence that the argument

is unsound. There is no reason to be convinced of the conclusion of an unsound

argument.

Remember

A proof of a contradiction ⊥ from premises P1, . . . ,Pn (without addi-

tional assumptions) shows that the premises are inconsistent. An argu-

ment with inconsistent premises is always valid, but more importantly,

always unsound.

Exercises

5.27.

Give two different proofs that the premises of the above argument are inconsistent. Your first

should use proof by cases but not DeMorgan’s law, while your second can use DeMorgan but

not proof by cases.

Chapter 5


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