Methods of Proof for Boolean Logic


 Clifton Ray
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1 Chapter 5 Methods of Proof for Boolean Logic limitations of truth table methods 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 truthfunctional connectives, but the method has two very significant limitations. First, truth tables get extremely large as the number of atomic sentences goes up. An argument involving seven atomic sentences is hardly unusual, but testing it for validity would call for a truth table with 2 7 = 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 truthfunctional 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 extensibility. 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
2 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 legitimate 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 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 wellknown 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 important rule of thumb Section 5.1
3 130 / Methods of Proof for Boolean Logic conjunction elimination (simplification) conjunction introduction disjunction introduction 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. This 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 to 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 should 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, capitalization, 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
4 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 mathematics, 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 properties. 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 significant 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 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. knowing your audience Remember 1. In giving an informal proof from some premises, if Q is already known to be a logical consequence of sentences P 1,..., P n and each of P 1,..., P n 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 understood. 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 unmentioned in informal proofs: From P Q, infer P. From P and Q, infer P Q. From P, infer P Q. Section 5.1
5 132 / 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 From P Q and P, infer Q. 5.2 From (P Q), infer P. 5.4 From P Q and Q, infer P. 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 wellknown piece of mathematical reasoning. The reasoning proves that there are irrational numbers b and c such that b c 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 section, to illustrate proof by contradiction; for now, just take it as a wellknown truth.) Here now is our proof: Proof: To show that there are irrational numbers b and c such that b c is rational, we will consider the number 2 2. We note that this number is either rational or irrational. Chapter 5
6 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 b c : b c = ( 2 2) 2 = 2 ( 2 2) = 2 2 = 2 Thus, we see that in this case, too, b c is rational. Consequently, whether 2 2 is rational or irrational, we know that there are irrational numbers b and c such that b c 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 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 P 1... P n, then we can break into n cases. In the first we assume P 1, in the second P 2, 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 proof by cases (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
7 134 / 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, stepbystep proof would look like this: Proof: Assume the disjunction: (Home(max) Happy(carl)) (Home(claire) Happy(scruffy)) Then either: or: Home(max) Happy(carl) 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 conclusion 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
8 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 conclusion 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 P 1... P n using this method, prove S from each of P 1,..., P n. Exercises The next two exercises present valid arguments. Turn in informal proofs of the arguments validity. Your proofs should be phrased in complete, wellformed English sentences, making use of firstorder 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
9 136 / 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) 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. 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. 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 wellknown burglar, Julius, who weighs two hundred and fifty pounds and is arthritic. Explain their reasoning. In our proof that there are irrational numbers b and c where b c 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? Describe an everyday example of reasoning by cases that you have performed in the last few days. Give an informal proof that if S is a tautological consequence of P and a tautological consequence 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
10 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 contradiction. 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 P 1,..., P n. One way to do this is by temporarily assuming 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 consequence of the original premises. Why? Because your proof of the contradiction shows that S, P 1,..., P n cannot all be true simultaneously. (If they were, the contradiction would have to be true, and it can t be.) Hence if P 1,..., P n are true in any set of circumstances, then S must be false in those circumstances. Which is to say, if P 1,..., P n 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). indirect proof or proof by contradiction 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 contradicts 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 n 2 is an even number, then so is n. And from this, we see that if n 2 is even, it must be divisible by 4. Now we re ready for the proof that 2 is irrational. Section 5.3
11 138 / 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: p 2 q 2 = 2 Multiplying both sides by q 2, we get p 2 = 2q 2. But this shows that p 2 is an even number. As we noted before, this allows us to conclude that p is even and that p 2 is divisible by 4. Looking again at the equation p 2 = 2q 2, we see that if p 2 is divisible by 4, then 2q 2 is divisible by 4 and hence q 2 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. contradiction 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 any 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 shorthand way of saying that a contra 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 demonstrating 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. contradiction symbol ( ) Chapter 5
12 Indirect proof: proof by contradiction / 139 Similarly, the truth table method gives us a way of showing that a collection of sentences are mutually contradictory. Construct a joint truth table for P 1,..., P n. These sentences are ttcontradictory if every row has an F assigned to at least one of the sentences. If the sentences are ttcontradictory, 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 already 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 encountered 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 reasoning 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 coworkers. 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 something 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 unacceptable. 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. ttcontradictory Remember Proof by contradiction: To prove S using this method, assume S and prove a contradiction. Section 5.3
13 140 / Methods of Proof for Boolean Logic Exercises In the following exercises, decide whether the displayed argument is valid. If it is, turn in an informal proof, phrased in complete, wellformed English sentences, making use of firstorder 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, construct a counterexample world in Tarski s World. (Argument 5.16 is valid, and so will not require a counterexample.) 5.15 b is a tetrahedron. c is a cube. Either c is larger than b or else they are identical. b is smaller than c 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 Cube(a) Tet(a) Large(a) Cube(a) a = b Large(a) Large(a) a = c (c = c Tet(a)) 5.18 Cube(a) Tet(a) Large(a) Cube(a) a = b Large(a) Large(a) a = c (c = c Tet(a)) a = b a = c (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 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
14 Arguments with inconsistent premises / Describe an everyday example of an indirect proof that you have used in the last few days. Prove that indirect proof is a tautologically valid method of proof. That is, show that if P 1,..., P n, S is ttcontradictory, then S is a tautological consequence of P 1,..., P n. 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 Assume that n 2 is odd. Prove that n is odd Assume that n + m is odd. Prove that n m is even Assume that n 2 is divisible by 3. Prove that n 2 is divisible by 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 consequence 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 circumstances 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 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 always valid Section 5.4
15 142 / Methods of Proof for Boolean Logic never sound much on its own. After all, the reason we are interested in logical consequence 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 P 1,..., P n (without additional assumptions) shows that the premises are inconsistent. An argument 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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