|
|
Line 1: |
Line 1: |
− | 1. If Socrates influenced Plato, then Plato's philosophy has some features of Socrate' thinking.
| |
− | 2. Socrates influenced Plato.
| |
− | C. Plato's philosophy has some features of Socrates' thinking.
| |
− | Notice that both of these arguments use patterns and that it is the patterns themselves that make the arguments work. Logic begins with the realization that the connections among ideas that lead to worthwhile inferences
| |
− | 102
| |
− |
| |
− |
| |
| 3.2 Deductive Logics: Categorical and Propositional. | | 3.2 Deductive Logics: Categorical and Propositional. |
− | When one of the company said to him, "Convince me that logic is necessary," "Would you have me demonstrate it to you?" he said. "Yes." Then I must use a demonstrative form of argument. "Granted." "And how will you know, then whether my arguments mislead you?" On this, the man being silent, Epictetus said, "You see that even by your own confession, logic is necessary; since without it, you cannot even learn whether it is necessary or not." from Epictetus, Enchiridion | + | :When one of the company said to him, "Convince me that logic is necessary," "Would you have me demonstrate it to you?" he said. "Yes." Then I must use a demonstrative form of argument. "Granted." "And how will you know, then whether my arguments mislead you?" On this, the man being silent, Epictetus said, "You see that even by your own confession, logic is necessary; since without it, you cannot even learn whether it is necessary or not." from Epictetus, Enchiridion |
| | | |
| + | You already have an intuitive idea of what logic is -- it is the study of reasoning that depends upon the way ideas are connected in patterns. But to go further, we need a more precise definition. Consider the following two arguments: |
| | | |
− | You already have an intuitive idea of what logic is -- it is the study of reasoning that depends upon the way ideas are connected in patterns. But to go further, we need a more precise definition. Consider the following two arguments:
| |
− | 1. All Italians are human.
| |
− | 2. All humans are mortal.
| |
− | C. All Italians are mortal.
| |
| | | |
− | 1. If Socrates influenced Plato, then Plato's philosophy has some features of Socrate' thinking. | + | :1. All Italians are human. |
− | 2. Socrates influenced Plato. | + | :2. All humans are mortal. |
− | C. Plato's philosophy has some features of Socrates' thinking. | + | :C. All Italians are mortal. |
| + | :1. If Socrates influenced Plato, then Plato's philosophy has some features of Socrate' thinking. |
| + | :2. Socrates influenced Plato. |
| + | :C. Plato's philosophy has some features of Socrates' thinking. |
| Notice that both of these arguments use patterns and that it is the patterns themselves that make the arguments work. Logic begins with the realization that the connections among ideas that lead to worthwhile inferences | | Notice that both of these arguments use patterns and that it is the patterns themselves that make the arguments work. Logic begins with the realization that the connections among ideas that lead to worthwhile inferences |
| + | 102 |
| | | |
| can be generalized so that we can focus on the connections or patterns of ideas in isolation from the content. To see this, we will rewrite the two arguments above replacing their content with placeholders. | | can be generalized so that we can focus on the connections or patterns of ideas in isolation from the content. To see this, we will rewrite the two arguments above replacing their content with placeholders. |
Line 138: |
Line 132: |
| Puzzling out which of the 256 standard form categorical syllogisms follow the rules of validity is probably not a worthwhile use of your time even if your goal is to become a better thinker.' The essential skill to take away from our brief survey of categorical logic is the ability to recognize how immediate inferences (obversion, conversion, and contraposition) and syllogistic inferences can be valid or invalid. By picturing the logical relationships among categories, you should become more confident of finding reliable and unreliable patterns of reasoning. Try out your skills on the following exercise set. | | Puzzling out which of the 256 standard form categorical syllogisms follow the rules of validity is probably not a worthwhile use of your time even if your goal is to become a better thinker.' The essential skill to take away from our brief survey of categorical logic is the ability to recognize how immediate inferences (obversion, conversion, and contraposition) and syllogistic inferences can be valid or invalid. By picturing the logical relationships among categories, you should become more confident of finding reliable and unreliable patterns of reasoning. Try out your skills on the following exercise set. |
| Exercise: Determine the validity or invalidity of the following arguments. With the arguments in part A (given in syllogistic form), apply the rules for determining validity. In part B, you are given arguments that could be translated into categorical form, but you should try to determine validity simply by picturing the logical relationships in the premises. | | Exercise: Determine the validity or invalidity of the following arguments. With the arguments in part A (given in syllogistic form), apply the rules for determining validity. In part B, you are given arguments that could be translated into categorical form, but you should try to determine validity simply by picturing the logical relationships in the premises. |
− | 3.4 Propositional Logic
| |
− | While Aristotle's categorical logic was based on the logical relationships that hold among categories, propositional logic focuses on relationships among propositions -- claims that are either true or false. Because we are still doing logic, we need to abstract from the actual content of arguments to see logical structure. Therefore, we will take a passage such as this
| |
− | If Saskwatch is real, then many people would have seen him over the years. Not many people have seen him, therefore I do not think he's real.
| |
− | And symbolize it like this: 1. If S, then P.
| |
− | 2. Not P.
| |
− | C. Not S.
| |
− | ' Medieval students were given a mnemonic to remember the 15 valid forms. Each of them was represented by a name whose vowels corresponded to the mood of the syllogism. So, "Barabara" stood for an "AAA" syllogism in the first mood (M-P, S-M, S-P). Spending this much energy memorizing forms makes sense as long as you imagine that the way to think well involved translating ordinary arguments into syllogisms and then evaluating them.
| |
− |
| |
− | In this symbolization, S stands for "Sasgwatch is real," and P stands for "People would have seen him." We will discuss the logic of some natural language expressions -- more than we did with categorical logic -- but our general goal is the same. We want to refine our ability to recognize valid argument patterns. In the pattern above, the truth of the premises guarantees the truth of the conclusion, so the pattern is valid. Whether your arguing about Sasquatch or taxes, if your argument fits that pattern and the premises you offer are true, then the conclusion given in the pattern will be true.
| |
− | Before we discuss more argument patterns, we should introduce the basic statement types in our propositional logic system. The statement types are determined by a few different types of logical connectives: "and," "or," "if . . . then", and "not", which are represented in logic by the symbols: . , V , -> , and -. All of these connectives join two propositions, usually symbolized by P, Q, R, and so on, except the negation symbol, called a "tilde," which simply negates a single expression. Since the propositions are either true or false, the expression created by the connective (e.g. P . Q can be understood as a "function" of the various combinations of truth and falsity of the individual propositions. We call these the "truth table" or "truth functional" definitions for each connective and we will explain them as we proceed.
| |
− | Because language is so much richer and more complex than terms in a logic (logicians would say that it is more vague and ill-defined), the correspondence between the connective "v" and the natural language expression "or" is not precise, so we need to discuss such differences for all of the connectives. In general, logical systems work by having clear and precise definitions for terms. Becoming more aware of the difference between the connectives in natural language and logical systems can help you avoid ambiguities in your speech and writing that might cause confusion. In some cases, however, it is precisely the richness and diversity of meaning in natural language that allow reflection to go forward, so we will not give up language for logic, even if we have much to learn from it.
| |
− | The conjunction is the simplest connective and it maps onto English in the most straightforward way. Conjunctions, in which we assert two propositions with an "and" between them are true as long as both propositions
| |
− |
| |
− | are true. If I say "I'm going to the river, and I'm going swimming" and then you later find out that I went to the mall or I went to the river, but did not swim, or I went swimming but not in the river, you would say that what I said earlier wasn't true. The truth value of the "." (°and")is defined in the table below for all possible combinations of true and false propositions.
| |
− | Truth Table Definition of "." (Conjunction) P Q P. Q
| |
− | T T T T F F F T F
| |
− | F F F
| |
− | Negations are also pretty straightforward. If the proposition "P" is true, then the proposition "not-P" is false. Of course, confusions are still possible. They usually occur when people try to get you to assign a "true or false" label to a complex statement, one that may not be simply true or false. Imagine that you're trying to choose a leader for an organization and someone ask you, "Do you think Mary would make a good president?" This sounds like a simple yes/no or true/false question. If you say yes, then you must think the proposition, "Mary would make a good president" is true. Then the negation of that proposition is false. But suppose you respond by saying "She wouldn't be a bad president," or "I'm not sure," or "Mary would be a good president in three years, after Ed serves a term." Now you are not simply saying "yes" or "no." These responses involve contexts in which a "bivalent" -- yes/no or true/false -¬assumption is intended in the question, but rejected in the answer. Logical systems, especially simple logical systems, are more effective as formal systems if they can abstract from complexities like these.
| |
− | As long as you have genuinely true/false propositions, then the truth value of a negation is opposite the true value of the proposition. Unlike the
| |
− |
| |
− | other connectives, negation only takes one expression, so its truth table is shorter.
| |
− | Truth Table Definition of "-" (Negation) P -P
| |
− | T F
| |
− | F T
| |
− | With disjunction, matters become more complex. Consider the circumstances under which the following statements, in the situations described, are true:
| |
− | 1. On a menu: "All gut-buster entrees come with your choice of soup or salad."
| |
− | 2. On your university's website: "Students who are planning to graduate in May or think they may be qualified to graduate in May should contact the Registrar for degree evaluation by January 15'"." 3. On a sign in front of a swimming area of a lake: "Pets or glass containers are not allowed in the swimming area."
| |
− | In the first statement, the meaning is clearly "one or the other but not both." But the meaning of "or" in the second example is not quite the same. It is a safe bet that many students planning to graduate in May also think they are qualified to graduate. If you said to yourself that the rule was for people who met one or the other condition but not both, and therefore you didn't contact the register, most people would say that you just misread the rule. That is because in this context "or" means "one or the other or both". The first example uses what is called "exclusive or" and the second uses "inclusive or," because it involves the possibility that this proposition is true if both conditions are met. Example number 3 is inclusive as well, but because it involves a negation ("not permitted"), it is an inclusive "neither . . . nor". If someone asked, "Are you following the rule," you could only answer "yes" if you had neither glass nor
| |
− |
| |
− | pets with you. These meanings of "or" produce propositions that are true or false under different circumstances. Usually, the context of communication makes the meaning clear, but in logic, where we abstract from context, we need to pick one meaning of "or" to work with. In our treatment of disjunction in propositional logic we will use "inclusive or." In a formal system of logic, we can express "exclusive or" by a more complicated expression.
| |
− | Truth Table Definition of "v" (Disjunction)
| |
− | P Q PvQ
| |
− | T T T
| |
− | T F T
| |
− | F T T
| |
− | The logical pattern "If P, then Q" is sometimes referred to as the "conditional" or "hypothetical." Its relationship to natural language meaning is the most complex of the four connectives. Consider some of the following examples:
| |
− | 1. If you get the EZ-Ski package, you get three ski lessons, equipment rental and three ski passes for $90. (definitional)
| |
− | 2. If you fire your clay in a hot kiln, it will harden. (prediction, causal)
| |
− | 3. If Sally pitches, we will win. (prediction, causal)
| |
− | 4. If you fail to respond to this traffic summons, a warrant will be issued for your arrest. (general consequential, causal)
| |
− | 5. If you say you will help her, you should do it. (moral, decisional, definitional)
| |
− | 6. If you do that one more time, I'll hit you. (threat, decisional)
| |
− | 7. If she goes on a date with you, I'll eat my hat. (expression of incredulity, negative prediction)
| |
− | 8. If it makes you happy, why the hell are you so sad? (challenge, question)
| |
− |
| |
− | Our goal here is the same as it was for the other connectives -- to understand how we should think about the relationship between the truth or falsity of each part of the connective (in this case, the "if" clause taken separately from the "then" clause) and the truth or falsity of the whole. In conditionals, we call the "if" clause the antecedent and the "then" clause the consequent.
| |
− | In the case of number 8, the popular lyric, it is not clear there is any truth value for the whole. Questions are moves in a conversational strategy. They are neither true nor false. Of course, that is not to deny that questions convey propositional content. Questions can contain assumptions or imply other claims. Similarly, it is not clear that number 7 really has an "if . . . then" meaning. We usually understand expressions like this to mean, "I doubt she will go on a date with you".
| |
− | A core meaning of many uses of "if ... then" is that the whole expression is true as a result of a connection between the truth of the "if" clause and the "then" clause. The hard thing to figure out is how we should evaluate the conditional if the antecedent does not obtain or if the consequent turns out to be true, but not as a result of the truth of the antecedent. Most people would say, after reading number 1, that if you don't get the EZ-Ski package, you won't get the lessons, etc, and that would be a reasonable inference in this context. We will see later that from a purely formal standpoint, this is a fallacious way to reason. Perhaps there is a conversational implication in number 1 that that is the only way to get everything it promises for that price? Notice that that isn't implied by our causal predictions. If you don't fire your clay, it might still harden for some other reason. Would you evaluate the whole expression as true in the case of someone who found another means of hardening their pot (maybe by freezing it!), or indeterminate? It might still be true that if you had fired it in a hot kiln, it would have hardened.
| |
− | Even if you respond to the traffic summons, you might be the subject of a warrant for arrest for some other reason. Would you consider conditional number 4 true in that case? Or would you say that the truth value cannot be determined because you did not test the conditional by failing to reply to the traffic summons? (You could also say that number four is not really a causal
| |
− |
| |
− | prediction at all, but just a statement of the rule governing people who do not respond to traffic summons.)
| |
− | Suppose you make the prediction, "If Sally pitches, we will win" and then she pitches very poorly, but the team wins anyway. Someone could reasonably say that your prediction was defective even though both the antecedent and consequent were true.
| |
− | As confusing as this topic can be, a couple of points can be drawn out from the discussion. First, some conditionals in everyday discourse do not really mean to implicate an "if ... then" logic. That's what we saw in numbers 7 and 8. Second, some conditions really involve a stronger set of conditions than the simple "if ... then" structure suggests. If you corrrectly infer that the conditional goes in both directions (If P, then Q AND If Q, then P), and this often happens with definitions and exclusive ski packages, then you really have a "biconditional" on your hands. We say the truth conditions for bi-conditionals are "stronger" than for conditionals because more conditions must be satisfied for the conclusion to be true.
| |
− | The big remaining problem is what to do with cases in which the conditional as a whole seems "undetermined" either because the antecedent did not obtain or because both parts were true, but not related to each other. . The key thing in these cases is to keep in mind the difference between actual reasoning in real situations, which is governing by the logic implicit in the actual meanings of our communication, and the needs of a logician creating a logical system in which all of the possible truth values for an expression must have a determinate outcome. For a logician, the relationships among propositions have to be investigated without any reference to the content of the propositions. The logician cannot leave any lines of the truth table blank, but real reflective practitioners can. With this in mind, we can define the logical connective "if... then" in terms of its truth table. The truth functional definition of the conditional is arrived at by finding the weakest or minimal logical meaning of "if ... then" and then relying on more elaborate translations of everyday meanings that say more or something more subtle. Following this way of thinking, the truth table for conditional is filled in as follows
| |
− |
| |
− | Truth Table Definition of "4" (Conditional) P Q P4Q
| |
− | T T T T F F F T T F F T
| |
− | We are almost ready to introduce the valid patterns of reasoning for propositional logic, but we do need to comment on the logic of a few specific English language terms. To start with a couple of easy ones, if,someone writes, "Smith's views on crime imply that he would support the death penalty," we could translate that as "S 4 D." 8 Also, "You may park in this lot provided that you have a green parking sticker," translates as "G 4 P."
| |
− | The translation of "only if" is somewhat more confusing. To translate "Your car will run only if it has gas," you might think that because the "if" appears right before "it has gas" the conditional is "G 4 R", roughly, "if it has gas, it will run". But that would be saying that "having gas" is a sufficient condition for the car to run, and that's probably not what a speaker of this sentence means. After all, any number of mechanical problems can keep a car with a full tank from running. The sentence meaning is "If it doesn't have gas, then it won't run," which is logically equivalent to "If it runs, then it has gas." ("-Q -> -P" is logically equivalent to "P -> Q").
| |
− | We will talk more about "necessary conditions" and "sufficient conditions" when we discuss causality, but since they relate to the conditional, we should also explain their translation. If some claim "P" is a "sufficient condition" for "Q", then it is translated "If P is true, then Q is true," which is "P 4 Q". But if something is a "necessary condition" then we are saying that if it doesn't obtain, then other claims won't be true. And that is what "-P 4 -Q" says,
| |
− | a In general, we will pick letters to represent independent clauses, using the first letter of a unique word in the clause.
| |
− |
| |
− | "If not-P, then not-Q." For the reason given above, this could be written as "Q 4 P11.
| |
− | Another common, and confusing, English expression that relates to the hypothetical is "unless". Consider the following two examples
| |
− | 1. We will go out Tuesday night unless Professor Timberlake sets the exam for Wednesday.
| |
− | 2. You are eligible for the deduction unless you are over 65 years of age.
| |
− | The first example seems to most naturally suggest that "if Professor Timberlake sets the exam for Wednesday then we will not go out," which translates as "E -> -O", but the context of communication would also usually support this interpretation: "If he doesn't set the exam, then we will go out," which translates as "-E ~ O." These are not logically equivalent, but they might both be implied by a speaker. When both of these conditions are implied, "unless" means the same thing as "or". So the first example also translates as "We will go out Tuesday or the exam is on Wednesday."
| |
− | The second example could also be translated, "If you are over 65, then you are not eligible for the deduction," and it might also mean, "If you are not over 65, then you are eligible." However, the context for "eligibility" discussions, especially involving taxes, is often complex. There is usually an understanding that there are other eligibility requirements. Depending upon the context, "P unless Q might not mean both "Q 4 -P" and "-Q 4 P", but typically we translate "unless" as "or".
| |
− | Valid Argument Patterns for Propositional Logic
| |
− | As with categorical logic, the valid argument patterns in propositonal logic are syllogisms (arguments with two premises) which reflect basic structures of reasoning. Five of the basic patterns are Modus Ponens, Modus Tollens, Disjunctive Syllogism, Hypothetical Syllogism and Double Negation. The patterns are summarized in the table below. We use lower case letters to signify that the propositions in these patterns are "sentential variables" and can be replaced by any particular sentence using an upper case letter to represent a particular sentence). The particular sentence is called a "sentential constant":
| |
− | 19
| |
− |
| |
− | Pattern Name
| |
− | Pattern Logical Features of the Pattern
| |
− | Modus Ponens
| |
− | Modus Tollens
| |
− | Disjunctive Syllogism
| |
− | Hypothetical Syllogism
| |
− | Double Negation
| |
− | p 4 q From any conditional, and the antecedent of the conditional, you may infer the consequent. q
| |
− | p 4 q From any conditional, and the negation of its consequent, you may infer the negation of its -p antecedent.
| |
− | p v q From any disjunction, and the negation of -) either disjunct, you may infer the other q disjunct.
| |
− | p 4 q From two conditionals in which the consequent
| |
− | a_ 4 r of one occurs as the antecedent of the other,
| |
− | p 4 r you may infer a conditional with the corresponding antecedent and consequent. --p From any double negated expression, you may
| |
− | p infer an unnegated expression. (And vice versa.)
| |
− | Notice that the third column of the table gives a natural language explication of the pattern. This statement of the patterns may seem cumbersome because it uses the vocabulary of "antecedent" and "disjunct", etc., however, this explanation is clearer and more complete than the pattern itself. You should read through these descriptions of our patterns carefully, paying attention to how the pattern should be applied to specific instances. For example, it is important to know that in a disjunctive syllogism, "either disjunct" may be negated and you can infer "the other". This is actually harder to show with the symbols, which necessarily pick one disjunct for the negation symbol.
| |
− | Many of these details are more relevant if you plan to learn the proof technique for propositional logic, which is given in supplemental material on the text website. Doing proofs in propositional logic is a good way to sharpen your eye for logical structure and validity, but it also takes us away from natural language arguments and the broader task of reflection, so we present it as an attractive detour, rather than as part of the main path.
| |
− |
| |
− | There are some general comments to make about the patterns, however. Notice that three of the five patterns concern "if. . .then" or conditional statements. If you know that the antecedent of a conditional is true, you can infer the consequent. If you know that the consequent is false, you can infer the negation of the antecedent. Finally, hypothetical syllogism establishes something like a "transitive property" for conditionals. just as you know that if x is greater than y, and y is greater than z, then x is greater than z, so too, if you know that the consequent of one conditional is linked to the antecedent of another, you can make a new conditional with the other two parts.
| |
− | Knowing the patterns should help you spot instances of them in natural language. Try to identify the pattern at work in the following premises. Then identify the conclusion that the pattern allows you to draw:
| |
− | 1. A: You can have strawberry or vanilla ice cream. B: I don't want vanilla.
| |
− | 2. If wishes were horses, pigs would fly. Pigs can't fly.
| |
− | 3. If the train arrives at seven, we will have time for dinner. The train arrived at seven.
| |
− | 4. If you mow the lawn, I'll do the laundry. If I do the laundry done, then we can go swimming.
| |
− | Try a few more in which two patterns are combined:
| |
− | 5. Shirley either loves me or she doesn't. If she loved me, she would call me on my birthday. But she didn't.
| |
− | 6. If astrology is a real science, we can find the tendencies of human behavior in the movements of the planets and stars. If people could find the tendencies of human behavior in the movements of the planets and starts, astrologers would be successful in predicting the future. Astrologers are not very successful in predicting the future.
| |
− | With this knowledge of propositional logic you can work on the skill of recognizing logical structure in short, semi-formalized passages. Untangling the premises and conclusions of these arguments and formalizing them can help you see logical structure in real natural language arguments.
| |
− | 21
| |
− |
| |
− | 3.5 Exercise: Formalizing Propositional Arguments
| |
− | The arguments in the following exercise set can be formalized into standard form propositional logic, but some of them require the addition of implied premises. Supply missing premises, rephrase claims and label them with individual letters, then write out the form of the argument. Then see if you can recognize the pattern or patterns at work in each deduction.
| |
− | Example:
| |
− | Unless he is a saint, a used car salesman will give in to the temptation to withhold information about the defects of the cars he sells. If he withholds information about used cars for sale, he will cause people to spend more for the cars than they are worth. Used car salesmen aren't saints.
| |
− | H - Used Car salemen are saints.
| |
− | T - Used Car salesmen withold information about the defects of the cars they sell.
| |
− | S - Used car salesmen cause people to spend more for the cars they buy. 1. HvT
| |
− | 2. TES 3. -H C. S
| |
− | 1. If the city should approve more funds for the garage, it will go bankrupt. If the garage goes bankrupt, businesses will close. If businesses close, the tax revenue for the city will go down. No one wants the city tax revenue to go down. The city should approve more funds for the garage.
| |
− | 2. "It is only about the things that do not interest one, that one can give a really unbiassed opinion; and this is no doubt the reason why an unbiassed opinion is always absolutely valueless." Missing Premise: If you are not interested in something, your opinion doesn't have value. [More Letters of Oscar Wilde, ed. Rupert Hart¬Davis. Cited in Kelly, The Art o f Reasoning]
| |
− | 22
| |
− |
| |
− | 3. Either the war on Iraq was justified or it wasn't. If it was, then we should continue to work for a stable democratic Iraq. If it wasn't, then we should pull out and apologize. If Iraq didn't have weapons of mass destruction, then the war wasn't justified. Iraq didn't have weapons of mass destruction. Therefore, we should pull out and apologize.
| |
− | 4. If people do not receive good information about sexually transmitted diseases, then they will not make good choices about their sexual health. If people do not make good choices about sex, them may come to negative associations with it. If people have negative associations with sex, them may become repressed. Therefore, if people do not receive good information about sexually transmitted deseases, they may become repressed.
| |
− | On a practical level, deductive reasoning is especially noticeable when you have a number of facts with a very clear logical structure. While you probably are not frequently in the position of Sherlock Holmes, Angela Landsbury (in "Murder She Wrote") or Peter Faulk (in "Columbo"), you should be able to recall the classic scence of a "who dunnit" in which the investigator puts all the pieces together. Suppose, for example, you know the following:
| |
− | 1. Smith was killed at his home with a large heavy object on the night of the 25th.
| |
− | 2. The cook, the butler, and the gardener are the only people who stay at the house with Smith.
| |
− | 3. The gardener takes the last week of the month off to visit his sister.
| |
− | 4. The cook burned his hands making creme broulee for a party on the 24"'.
| |
− | 5. The butler had been working out and bragging to people that Smith had left him a million dollars in his will.
| |
− | Well, things don't look good for the butler, but murder mysteries are not solved exclusively by deductive logic; or even by logic alone. You have to know something about human psychology and, in this case, the means of committing the murder. There are, however, deductive structures at work in our thinking
| |
− | 23
| |
3.2 Deductive Logics: Categorical and Propositional.
- When one of the company said to him, "Convince me that logic is necessary," "Would you have me demonstrate it to you?" he said. "Yes." Then I must use a demonstrative form of argument. "Granted." "And how will you know, then whether my arguments mislead you?" On this, the man being silent, Epictetus said, "You see that even by your own confession, logic is necessary; since without it, you cannot even learn whether it is necessary or not." from Epictetus, Enchiridion
You already have an intuitive idea of what logic is -- it is the study of reasoning that depends upon the way ideas are connected in patterns. But to go further, we need a more precise definition. Consider the following two arguments:
- 1. All Italians are human.
- 2. All humans are mortal.
- C. All Italians are mortal.
- 1. If Socrates influenced Plato, then Plato's philosophy has some features of Socrate' thinking.
- 2. Socrates influenced Plato.
- C. Plato's philosophy has some features of Socrates' thinking.
Notice that both of these arguments use patterns and that it is the patterns themselves that make the arguments work. Logic begins with the realization that the connections among ideas that lead to worthwhile inferences
102
can be generalized so that we can focus on the connections or patterns of ideas in isolation from the content. To see this, we will rewrite the two arguments above replacing their content with placeholders.
1. All S are M.
2. All M are P.
C. All S are P.
1. If A, then B. 2. A.
C. B.
In the first case we replaced every instance of "Italians" with S, "humans" with M, and "mortal" with P. In the second case we replaced every independent clause with letters A and B, making sure that the same clause was replaced with the same letter. The remarkable thing to notice now -- and you should try to imagine what it must have been like to be one of the first humans to notice it -¬is that the resulting patterns could be filled by any content (collections of things in the first argument, true/false claims in the second) and the reasoning would be sound. Historically, logicians were the first people who noticed this and wrote about it. We can generalize patterns of thought by abstracting content and studying the relationships among the patterns.
Notice one more thing about the argument patterns above. The first one is filled with terms, while the second one is filled with true/false propositions. The terms in the first argument refer to categories of things (concrete or abstract) and this logic is called "categorical logic." It was first codified by Aristotle. The second is about logical relationship among propostions and is called "propositional logic." The Stoics were the first thinkers in the West to write about these patterns, but propositional logic developed its modern form in the 19'" and 20"' centuries.
Taking categorical logic first, let's look at the basic set of relationships Aristotle discovered. He distinguished categorical propositions according to two distinctions:
2
1. Whether they predicate something (assert something) about everything in a category -- in which case they are universal -- or some particulars in a category and;
2. Whether they predicate something about a category -- in which case they are affirmative -- or deny it, and therefore are called "negative".
With these two distinctions we can sort categorical propositions into four groups:
Universal Affirmative: All S are P.
Universal Negative: No S are P.
Particular Affirmative: Some S are P.
Particular Negative: Some S are not P.
Even before we look at arguments with categorical logic, you can see that there are truth relationships among different types of propositions. For instance, if "All Humans are Mortal" is true (and it is) then "No Humans are Mortal" is false. We call these relationships "truth functional" because the truth of the proposition is a function (can be determined according to a rule) of the other.
The "Square of Opposition" is a traditional schema for summarizing the primary truth functional relationships among the four types of categorical propositions. Notice that we refer to each type of categorical proposition with a unique letter A, E,1, or O.
The Square of Opposition
A: All S are P.
N
Contraries 1 E: No S are P.
Subalterns Contradic£ories Subalterns
2: Some S are P.
Subcontraries L 0: Some S are not P.
The following relationships hold between categorical statements in the square of opposition:
Contradictories: Contradictories have opposite truth values. If one is true, the other must be false.
Contraries: Contraries cannot both be true, but can both be false. Subcontraries: Subcontraries cannot both be false, but can both be true.
Subalterns: From a true A or E proposition, you can infer a true I or 0 proposition.'
There are some additional truth functional relationships within categorical propositions, but they involve logical operations that can be carried out on particular propositions. For instance, you can "take the converse" of a proposition by switching its subject and predicate. So "All S are P" becomes "All P are S," "No S are P," becomes "No P are S," and so one. Conversion only works ("works" here means "preserves the truth value of the original proposition in the resulting one) on E and I propositions. It is worth noting that in everyday reasoning people sometimes reason from an A proposition to its converse or from an O proposition to its converse. Consider the following examples for each statement type:
1. All bachelors are unmarried males. All unmarried males are bachelors.
2. All students are members of the university community. All members of the university community are students.
3. Some professors are not philosophers. Some philosophers are not professors.
4. Some students are not physics majors. Some physics majors are not students.
' The relationship between subalterns only holds if you assume that the subject and predicate terms actually exist. This is called the existential interpretation of categorical logic. Aristotle made this assumption because he felt that when we assert something we should always be talking about something real. Modern logics often make different assumptions about existential import, but we will stick with Aristotle's approach.
4
Examples 1 and 3 both have pairs of true claims, but examples 2 and 4 show that for A and O propositions you can have a true claim whose conversion is not true. This means that the inference from and A or O proposition to its converse is not valid. It may seem like a technical point, but there is a very specific practical lesson in it. Whenever someone argues that it "just follows" from a statement of one logical form (A, E, 1, O) to another, you should think a moment about whether the conclusion they are advancing really follows as a matter of logic. To test this, try to come up with a counterexample in which the premise is true and the conclusion is false. If you can find such a counterexample, then the inference pattern is not valid.
You can also take the obverse of a proposition by changing it from affirmative to negative (or negative to affirmative) and then changing the predicate to its complement. The complement of P is non-P, the complement of "dogs" is "non-dogs" (everything in the universe that isn't a dog), and so on. Now we can ask the same question as with conversion: Which of the following inferences are valid?
1. All S are P. Therefore, no S are non-P.
2. No S are P. Therefore, all S are non-P.
3. Some S are P. Therefore, some S are not non-P.
4. Some S are not P. Therefore, some S are non-P.
The obverse of a true A, E, I, or O statement is always true. You will not find a counter-example.
Let's consider one final operation: contraposition. In this case we would reason, for example, from "All skiers are thrill seekers" to "All non-thrill seekers are non-skiers." With contraposition we switch the subject and predict within each proposition and put in their complements. This produces valid inferences for A and O propositions but not for E and I propositions.
Very few people -- even very few philosophers -- remember which propositions conversion, obversion, or contraposition work with or even the exact operation of each by name. It is still valuable to think through the operations, however. The real lesson here may not be obvious, so perhaps we should underscore it. In actual argument contexts, you may want to argue that an immediate inference can be made from a proposition with one logical form
5
to another. To evaluate the validity of that inference, you have two choices. You can hope that you remember the operation and which propositions it is valid for or you can try to figure out, on the spot, whether the inference is valid. To test the inference, you can try to come up with a counter-example in which you find a true premise which produces a false conclusion, as we did with obversion. Or, you can try to picture the class relationship itself by trying to think about whether the premise creates divisions and connections among the categories of things that necessarily includes the situation pictured in the conclusion. Try this with the example above about skiers and thrill seekers and you may find that you can "see" the logical relationship in the premise in such a way that makes it immediately apparent that the conclusion must be true.
Exercise Set: Practice with immediate inference. In the first part of the exercise below, determine whether the immediate inference being made is converse, obverse, or contraposition and whether it is a valid inference. In the second part, symbolize the sentence given and perform the indicated operation. Then determine whether the resulting inference is valid.
A.
1. All S is P. Therefore, All P is S.
2. Some S is P. Therefore, Some non-P is non-S. 3. No S is P. Therefore, all P is non-S.
4. Some S is not P. Therefore, some S is non-P. 5. Some S is P. Therefore, some P is S.
B.
1. All students are scholars. Converse. 2. Some melons are ripe. Obverse.
3. No children allowed near the bar. Contraposition. 4. Some people are dangerous. Obverse.
5. All garage bands are loud. Obverse.
3.3 Categorical Syllogisms
One goal for developing a logical system is to understand better the logical structure of particular statements and how changing those statements affects their truth value. But we also want our logical system to tell us about
6
the validity of complete arguments, not just immediate inferences from individual statements. In categorical logic, the argument form is typically a syllogism, an argument with two premises and, of course, a conclusion. One way to distinguish different syllogisms is by the combination of the four proposition types used. For example, you could represent a syllogism with all "A" propositions as "AAA" and one with "A" proposition premises and a particular affirmative conclusion as "AAI". The combination of proposition types in a categorical syllogism is called the mood of the syllogism and there are 64 different moods.
Another way syllogisms differ from one another is in the pattern of their subject and predicate terms. Consider the following two "AII" syllogisms:
1.
1. All P is M.
2. Some S are M.
C. Some S are P.
2.
1. All M is P.
2. Some Mare S. C. Some S are P.
Notice that while these syllogisms have the same mood, their middle terms -- the term or category that is repeated -- are in different places in the premises. There are two more positions they could have been in as well -- The middle terms could have been split between the subject and predicate terms of the major and minor premises as follows:
3.
1. All MisP. 2. Some S are M. C. Some S are P.
4.
1. All PisM.
2. Some Mare S. C. Some S are P.
7
These four possible configurations of the middle term relative to the subject and predicate terms are called the four figures. Since there are four figures for each of the 64 moods of categorical syllogism, there are are 256 syllogist forms in classical syllogistic logic.
The obvious question is, "Which of the 256 syllogistic forms are valid?" There are several approaches we could use to determine this. We could inspect each form and try to develop a counter-example for it or a reason for assuring ourselves that it is valid. We could use a method of picturing the relationships in the premises using Venn Diagrams .6 Or, we could rely on the work of 2,000 years of logicians who have developed three handy rules for evaluating a syllogism. Since our goal in learning a bit about categorical logic is primarily to pick up some critical awareness of logical form and validity, we will use the third method, though your instructor may choose to introduce Venn Diagrams instead or in addition.
If you were studying logic in the West at any time up through the 18 '" century, you probably would have been required to memorize the 15 valid forms of the the categorical syllogism. While logic is no less important a discipline than it ever (indeed, given the reliance of human beings on circuits and computer programs, logic is a matter of life and death for millions of people every day!), our development of the art of reflection depends less upon memorizing forms and rules than upon understanding the way logical structures create valid patterns.
To understand the three rules for evaluating categorical syllogisms, we need to introduce one more concept, the idea of "distribution." A term in a categorical proposition is distributed if the proposition says something about every member of the class named by that term. So, for example, in "All S is P," something is said about every member of the category named in the subject term, S, but not every "P". Even if Pi: "All male park rangers are handsome" is true, it would not follow that "All handsome people are park rangers. Nothing is being said about all "handsome people" in P,. However, in "No S is P"
I Venn Diagrams date from the work of John Venn, a British logician who introduced the Venn Diagram as a pictorial method for representing logical relationships. For more resources on Venn Diagrams, visit the textbook website.
8
something is said about every member of both classes. Can you see why? In the case of an "I" proposition, we are only asserting that some S's are P, so the subject term is not distributed. Further, we are not saying anything about all of the P's.
Determining whether the terms in the last propositon type, the "O" proposition, are distributed is a little trickier. We are not saying anything about all of the members of the category "S" in "Some S are not-P", so the subject term is not distributed, but we are actually saying something about all of the members of the category "P" -- every member of P is not one of the S's designated by "Some S". For example, if I say, "Some cars are not gas-powered," then I am asserting of all gas powered vehicles that they do not include those particular cars mentioned in the subject term. To summarize, in the following diagram the distributed terms are underlined:
Summary of Distributed Terms in
Categorical Propositions A: All S is P.
E: No S is P. I: Some S is P. 0: Some S is not P.
(Distributed terms are underlined.)
With this in mind, the three rules for evaluating categorical syllogisms for validity can be stated as follows:
1. The number of negative claims in the conclusion must equal the number of negative claims in the premises.
2. At least one premise must distribute the middle term.
3. Any term that is distributed in the conclusion of the syllogism must be distributed in its premises.
As you can see, the concept of distribution plays a significant role in these rules. Let's look at a couple of valid and invalid syllogisms using these rules. First, consider this syllogism:
1. All S are M.
2. Some M are not P.
C. Some S are not P.
This syllogism complies with the first rule (it has one negative proposition in each part) and fits the third rule (P is distributed in both the conclusion and P2' but it does not distribute it's middle term, that is, the premises do not say something about all of the members of the category named by "M" in either premise. The argument pattern is therefore invalid.
Notice that we were able to determine distribution and validity without knowing anything about the actual content of the categories S, M, and P. This is an essential value of logic -- the ability to know from the form of an expression that it can or cannot guarantee true conclusions.
You could also determine the invalidity of this syllogism by thinking carefully about its form without the rules. Here an example will help. Suppose you used the following instance of the argument pattern:
1. All crew team members are my friends.
2. Some of my friends are people who are not good at math.
C. Some crew team members are people who are not good at math. Intuitively, you might notice that there's no reason to think that the friends refered to in premise 2 are crew team members. After all, the first premise does not say that "All my friends are crew team members." So when you go to say something about crew team members in the conclusion, you can be sure you're talking about friends, but not necessarily those friends mentioned in premise 2. Let's try changing the first premise so the argument reads as follows:
1. All my friends are crew team members.
2. Some of my friends are people who are not good at math.
C. Some crew team members are people who are not good at math.
The person who gives this argument may have a more limited social life, but at least they have a valid argument. Now that we know that all of this person's friends are crew team members, anything he says about his friends is also true of the crew team. Notice too that the rules are satisfied by this syllogism.
Puzzling out which of the 256 standard form categorical syllogisms follow the rules of validity is probably not a worthwhile use of your time even if your goal is to become a better thinker.' The essential skill to take away from our brief survey of categorical logic is the ability to recognize how immediate inferences (obversion, conversion, and contraposition) and syllogistic inferences can be valid or invalid. By picturing the logical relationships among categories, you should become more confident of finding reliable and unreliable patterns of reasoning. Try out your skills on the following exercise set.
Exercise: Determine the validity or invalidity of the following arguments. With the arguments in part A (given in syllogistic form), apply the rules for determining validity. In part B, you are given arguments that could be translated into categorical form, but you should try to determine validity simply by picturing the logical relationships in the premises.