Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments 2. The Logic of Compound Statements Aaron Tan 20 24 August 2018 1 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments 2. The Logic of Compound Statements 2.1 Logical Form and Logical Equivalence Statements; Compound Statements; Statement Form (Propositional Form) Logical Equivalence; Tautologies and Contradictions 2.2 Conditional Statements Conditional Statements; If-Then as Or Negation, Contrapositive, Converse and Inverse Only If and the Biconditional; Necessary and Sufficient Conditions 2.3 Valid and Invalid Arguments
Argument; Valid and Invalid Arguments Modus Ponens and Modus Tollens Rules of Inference Fallacies 2 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments 2. The Logic of Compound Statements At the end of this lecture, you should be able to solve this puzzle: You are about to leave for school in the morning and discover that you dont have your glasses. You know the following statements are true: a. b. c. d. e. If I was reading the newspaper in the kitchen, then my glasses are on the kitchen table. If my glasses are on the kitchen table, then I saw them at breakfast. I did not see my glasses at breakfast. I was reading the newspaper in the living room or I was reading the
newspaper in the kitchen. If I was reading the newspaper in the living room then my glasses are on the coffee table. So, where are your glasses? 3 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments 2. The Logic of Compound Statements Another puzzle! Mr Alton is looking at Ms Betty, but Ms Betty is looking at Mr Carl. Mr Alton is married, but Mr Carl is not. Is a married person looking at an unmarried person? A. Yes. B. No. C. Cannot be determined. Socractive app: Room P7PS9AB27 Out of 200,000 submissions 27.68% 4.55% 67.77% Yes
No Cannot be determined Touted as the logic question that almost everyone gets wrong. https://www.theguardian.com/science/2016/mar/28/did-you-solve-it-the-logic-que stion-almost-everyone-gets-wrong 4 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments 2.1 Logical Form and Logical Equivalence 5 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Example If Jane is a math major or Jane is a computer science major, then Jane will take MA1101R.
If CS1231 is easy or I______________, study hard then I_____________________. will get A+ in this course Jane is a computer science major. Therefore, Jane will take MA1101R. I study hard. Therefore, I will get A+ in this course. 6 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Statements 2.1.1. Statements If Jane is a math major or Jane is a computer science major, then Jane will take MA1101R. Jane is a computer science major. Statements
Therefore, Jane will take MA1101R. Definition 2.1.1 (Statement) A statement (or proposition) is a sentence that is true or false, but not both. 7 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Common Form If Jane is a math major or Jane is a computer science major, then Jane will take MA1101R. Jane is a computer science major. Therefore, Jane will take MA1101R. If p or q, then r. q. Therefore, r. Statement variables 8 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Compound Statements 2.1.2. Compound Statements Logical connectives:
~ also not Truth values: p T F ~p F T and or p q pq p q
pq T T F F T F T F T F F F T T F F T F T F T T T F 9
Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Compound Statements: Negation, Conjunction, and Disjunction Definition 2.1.2 (Negation) If p is a statement variable, the negation of p is not p or it is not the case that p and is denoted ~p. Definition 2.1.3 (Conjunction) If p and q are statement variables, the conjunction of p and q is p and q, denoted p q. Definition 2.1.4 (Disjunction) If p and q are statement variables, the disjunction of p and q is p or q, denoted p q. 10 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Compound Statements: Order of Operations Order of operations: ~ is performed first and are coequal in order of operation ~p q = (~p) q pqr
Ambiguous Use parentheses to override or disambiguate order of operations ~(p q) Negation of p q (p q) r p (q r) Unambiguous 11 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Compound Statements: Quick Quiz Given: h = It is hot s = It is sunny Write logical statements for the following: a. It is not hot but it is sunny. ~h s b. It is neither hot nor sunny. ~h ~s
or ~(h s) (we will discuss this later) 12 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Statement Form 2.1.3. Statement Form (Propositional Form) Examples: ~p q (p q) ~(p q) (p q) r Definition 2.1.5 (Statement Form/Propositional Form) A statement form (or propositional form) is an expression made up of statement variables and logical connectives that becomes a statement when actual statements are substituted for the component statement variables. 13
Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Evaluating the Truth of Compound Statements Construct the truth table for this statement form: (p q) ~(p q) p q pq pq ~(p q) (p q) ~(p q) T T T T F F
T F T F T T F T T F T T F F F F T
F (p q) ~(p q) is also known as exclusive-or (why?) Denoted as p q or p XOR q . 14 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Logical Equivalence 2.1.4. Logical Equivalence (1) Dogs bark and cats meow. (2) Cats meow and dogs bark. If (1) is true, it follows that (2) must also be true. On the other hand, if (1) is false, it follows that (2) must also be false. (1) and (2) are logically equivalent statements. 15
Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Logical Equivalence Definition 2.1.6 (Logical Equivalence) Two statement forms are called logically equivalent if, and only if, they have identical truth values for each possible substitution of statements for their statement variables. The logical equivalence of statement forms P and Q is denoted by P Q. Example: a T b T ab T ba T T F F F
T F F F F F F F a b and b a always have the same truth values, hence they are logically equivalent. 16 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Logical Equivalence: Double Negative Property Double negation: ~(~p) p p T F ~p F
T ~(~p) T F 17 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Logical Equivalence: Showing Non-equivalence To show that statement forms P and Q are not logically equivalent, there are 2 ways: Truth table find at least one row where their truth values differ. Find a counter example concrete statements for each of the two forms, one of which is true and the other of which is false. 18 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Logical Equivalence: Showing Non-equivalence
Show that the following 2 statement forms are not logically equivalent. ~(p q) ~p ~q Truth table method: p q ~p ~q pq ~(p q) ~p ~q T T F T F T F F T F
T F T F F F T T F F F F F T T F T T 19 Logical Form and Logical Equivalence Conditional Statements
Valid and Invalid Arguments Logical Equivalence: Showing Non-equivalence Show that the following 2 statement forms are not logically equivalent. ~(p q) ~p ~q Counter example method: Let p be the statement 0 < 1 and q the statement 1 < 0. ~(p q) Not the case that both 0<1 and 1<0 which is TRUE. ~p ~q Not 0<1 and not 1<0 which is FALSE. 20 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Logical Equivalence: De Morgans Laws De Morgans Laws: ~(p q) ~p ~q
~(p q) ~p ~q Write negations for each of the following: a. John is 6 feet tall and he weighs at least 200 pounds. John is not 6 feet tall or he weighs less than 200 pounds. b. The bus was late or Toms watch was slow. The bus was not late and Toms watch was not slow. or Neither was the bus late nor was Toms watch slow. 21 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Tautologies and Contradictions 2.1.5. Tautologies and Contradictions Definition 2.1.7 (Tautology) A tautology is a statement form that is always true regardless of the truth values of the individual statements substituted for its statement variables. A statement whose form is a tautology is a tautological statement. Definition 2.1.8 (Contradiction) A contradiction is a statement form that is always false regardless of the truth values of the individual statements substituted for its statement variables. A statement whose form is a contradiction is a contradictory statement. 22 Logical Form and Logical Equivalence
Conditional Statements Valid and Invalid Arguments Tautologies and Contradictions Logical equivalence involving tautologies and contradictions Example: If t is a tautology and c is a contradiction, show that: ptp p T F t T T and c F F pt T F pcc pc F F As t and c (used in the textbook) are hard to
distinguished from statement variables, we will use true and false instead. 23 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Summary of Logical Equivalences 2.1.6. Summary of Logical Equivalences Theorem 2.1.1 Logical Equivalences Given any statement variables p, q and r, a tautology true and a contradiction false: 1 Commutative laws pqqp pqqp 2 Associative laws (p q) r p (q r) (p q) r p (q r) 3 Distributive laws p (q r) (p q) (p r) p (q r)
(p q) (p r) 4 Identity laws p true p p false p 5 Negation laws p ~p true p ~p false 6 Double negative law ~(~p) p 24 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Summary of Logical Equivalences 2.1.6. Summary of Logical Equivalences Theorem 2.1.1 Logical Equivalences (continue) Given any statement variables p, q and r, a tautology true and a contradiction false: 7 Idempotent laws ppp ppp
8 Universal bound laws p true true p false false 9 De Morgans laws ~(p q) ~p ~q ~(p q) ~p ~q 10 Absorption laws p (p q) p p (p q) p 11 Negation of true and ~true false false ~false true 25 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Simplifying Statement Forms: Quick Quiz
Use the laws in Theorem 2.1.1 to verify the following logical equivalence: ~(~p q) (p q) p ~(~p q) (p q) (~(~p) ~q) (p q) De Morgans (p ~q) (p q) Double negative p (~q q) Distributive p (q ~q) Commutative p false Negation p Identity 26 Logical Form and Logical Equivalence
Conditional Statements Valid and Invalid Arguments 2.2 Conditional Statements 27 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional Statements 2.2.1. Conditional Statements If Jane is a math major or Jane is a computer science major, then Jane will take MA1101R. hypothesis conclusion If 4,686 is divisible by 6, then 4,686 is divisible by 3. Conditional statement If p, then q
pq 28 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional Statements Logical connective: Truth values: if-then/implies p T q T pq T T F F F T
F F T T Definition 2.2.1 (Conditional) If p and q are statement variables, the conditional of q by p is if p then q or p implies q, denoted p q. It is false when p is true and q is false; otherwise it is true. We called p the hypothesis (or antecedent) of the conditional and q the conclusion (or consequent). 29 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional Statements A conditional statement that is true by virtue of the fact that its hypothesis is false is often called vacuously true or true by default. p q pq T
T T T F F F T T F F T If you show up for work Monday morning, then you will get the job is vacuously true if you do NOT show up for work Monday morning. In general, when the if part of an if-then statement is false, the statement as a whole is said to be true, regardless of whether the conclusion is true or false. 30 Logical Form and Logical Equivalence
Conditional Statements Valid and Invalid Arguments Conditional Statements: Example #1 Example #1: A Conditional Statement with a False Hypothesis If 0 = 1, then 1 = 2 Strange as it may seem, the statement as a whole is true! 31 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional Statements: Order of Operations Order of operations: ~ not
and or if-then/implies Coequal in order Performed first Performed last 32 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional Statements: Example #2 Example #2: Truth Table for p ~q ~p p ~q ~p p conclusion q ~p
~q T T F T F T F F F F T T F T F T (p (~q)) (~p) hypothesis p ~q T T F T p ~q ~p F
F T T 33 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional Statements: Example #3 Example #3: Show that pqr p q r pq T T T T
T F T F T T F F F F T T F F T F T F F F T T T T T T
F F pr qr T F T F T T T T T F T T T F T T (p r) (q r) pqr (p r) (q r) T F T F T F T
T T F T F T F T T 34 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Representation of If-Then as Or 2.2.2. Representation of If-Then as Or Rewrite the following statement in if-then form: Either you get to work on time or you are fired. Let ~p be You get to work on time and q be You are fired. ~p q Also, p is You do not get to work on time. If you do not get to work on time, you are fired. pq 35 Logical Form and Logical Equivalence
Conditional Statements Valid and Invalid Arguments Representation of If-Then as Or p q pq p T T F T F T T F T T T F F F
T F q ~p q T T F F ~pT q T F T pq 36 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Negation of a Conditional Statement 2.2.3. Negation of a Conditional Statement In previous slide, we have shown pq
~p q Implication law ~(p q) ~(~p v q) ~(~p) ~q p ~q ~(p q) p ~q 37 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Negation of a Conditional Statement: Quick Quiz Write negation for each of the following statements: a. If my car is in the repair shop, then I cannot get to class. My car is in the repair shop and I can get to class. b. If Sara lives in Athens, then she lives in Greece. Sara lives in Athens and she does not live in Greece. 38 Logical Form and Logical Equivalence
Conditional Statements Valid and Invalid Arguments Contrapositive 2.2.4. Contrapositive of a Conditional Statement Definition 2.2.2 (Contrapositive) The contrapositive of a conditional statement of the form if p then q is if ~q then ~p Symbolically, The contrapositive of p q is ~q ~p. pq ~q ~p contrapositive 39 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Contrapositive: Quick Quiz Write each of the following statements in its equivalent contrapositive form: a. If Howard can swim across the lake, then Howard can swim to the island.
If Howard cannot swim to the island, then Howard cannot swim across the lake. b. If today is Easter, then tomorrow is Monday. If tomorrow is not Monday, then today is not Easter. 40 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Converse and Inverse 2.2.5. Converse and Inverse of a Conditional Statement Definition 2.2.3 (Converse) The converse of a conditional statement if p then q is if q then p Symbolically, The converse of p q is q p. Definition 2.2.4 (Inverse) The inverse of a conditional statement if p then q is if ~p then ~q Symbolically, The inverse of p q is ~p ~q. 41 Logical Form and Logical Equivalence Conditional Statements
Valid and Invalid Arguments Converse and Inverse Conditional statement: qp converse pq ~p ~q inverse 42 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Converse and Inverse: Quick Quiz Write the converse and inverse of the following statements: a. If Howard can swim across the lake, then Howard can swim to the island. Converse: If Howard can swim to the island, then Howard can swim across the lake. Inverse:
If Howard cannot swim across the lake, then Howard cannot swim to the island. b. If today is Easter, then tomorrow is Monday. Converse: If tomorrow is Monday, then today is Easter. Inverse: If today is not Easter, then tomorrow is not Monday. 43 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Conditional statement and its Contrapositive, Converse and Inverse pq ~q ~p contrapositive ~p ~q inverse qp
conditional statement qp converse Note that: pq 44 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Only If and the Biconditional 2.2.6. Only If and the Biconditional To say p only if q means that p can take place only if q takes place also. That is, if q does not take place, then p cannot take place. Another way to say this is that if p occurs, then q must also occur (using contrapositive). Definition 2.2.5 (Only If) If p and q are statements, p only if q means Or, equivalently, if not q then not p if p then q 45
Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Only If : Quick Quiz Rewrite the following statement in if-then form in two ways, one of which is the contrapositive of the other. John will break the worlds record only if he runs the mile in under four minutes. Version 1: If John does not run the mile in under four minutes, then John will not break the worlds record. Version 2: If John breaks the worlds record, then John will have run the mile in under four minutes. 46 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Only If and the Biconditional Definition 2.2.6 (Biconditional) Given statement variables p and q, the biconditional of p and q is p if, and only if, q and is denoted p q. It is true if both p and q have the same truth values and is false if p and q have opposite truth values.
The words if and only if are sometimes abbreviated iff. pq p q pq T T F F T F T F T F F T 47 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Only If and the Biconditional
pq p q T T F T F T F F pq qp T T F T T F T T (p q) (q p) pq T F
F (p q) (q p) T T T F F 48 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Only If and the Biconditional Order of operations: ~
not and or if-then/implies if and only if Coequal in order Coequal in order Performed first Performed last 49 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Biconditional : Quick Quiz Rewrite the following statement as a conjunction of two if-then statements. This computer program is correct if, and only if, it produces correct answers for all possible sets of input data. If this computer program is correct, then it produces correct answers for all possible sets of input data, and if this program produces the correct answers for all possible sets of input data, then it is correct.
50 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Necessary and Sufficient Conditions 2.2.7. Necessary and Sufficient Conditions Definition 2.2.7 (Necessary and Sufficient Conditions) If r and s are statements, r is a sufficient condition for s means if r then s r is a necessary condition for s means if not r then not s or if s then r In other words, to say r is a sufficient condition for s means that the occurrence of r is sufficient to guarantee the occurrence of s. 51 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Necessary and Sufficient Conditions On the other hand, to say r is a necessary condition
for s means that if r does not occur, then s cannot occur either: The occurrence of r is necessary to obtain the occurrence of s. Note that due to the equivalence between a statement and its contrapositive: r is a necessary condition for s also means if s then r. Consequently, r is a necessary and sufficient condition for s means r, if and only if, s. 52 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments 2.3 Valid and Invalid Arguments 53 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Valid and Invalid Arguments
2.3.1. Valid and Invalid Arguments Argument: a sequence of statements ending in a conclusion. If Socrates is a man, then Socrates is mortal. Socrates is a man. Socrates is mortal. Abstract form If p, then q p q An argument form is called valid if, and only if, whenever statements are substituted that make all the premises true, the conclusion is also true. 54 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Valid and Invalid Arguments Definition 2.3.1 (Argument) An argument (argument form) is a sequence of statements (statement forms). All statements in an argument (argument form), except for the final one, are called premises (or assumptions or hypothesis). The final statement (statement form) is called the conclusion. The symbol , which is read therefore, is normally placed just before the conclusion. To say that an argument form is valid means that no matter what particular statements are substituted for the statement variables in its premises, if the
resulting premises are all true, then the conclusion is also true. Example: If p, then q p q premises conclusion 55 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Valid and Invalid Arguments When an argument is valid and its premises are true, the truth of the conclusion is said to be inferred or deduced from the truth of the premises. If a conclusion aint necessarily so, then it isnt a valid deduction. 56 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments
Determining Validity or Invalidity 2.3.2. Determining Validity or Invalidity Testing an Argument Form for Validity 1. Identify the premises and conclusion of the argument form. 2. Construct a truth table showing the truth values of all the premises and the conclusion. 3. A row of the truth table in which all the premises are true is called a critical row. If there is a critical row in which the conclusion is false the argument form is invalid. If the conclusion in every critical row is true the argument form is valid. 57 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Determining Validity or Invalidity: Example #1 p q ~r qpr Invalid argument pr premises conclusion
p q r ~r q ~r pr p q ~r qpr T T T F T T T T T
T F T T F T F T F T F F T F T T F
F T T F T T F T T F T F T F F T F
T T F T F F F T F F F T T F F F T
T F T T pr T Critical rows F T T 58 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Modus Ponens and Modus Tollens 2.3.3. Modus Ponens and Modus Tollens Syllogism: An argument form consisting of two premises and a conclusion. A famous form of syllogism is called modus ponens: If p then q
p q 59 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Modus Ponens and Modus Tollens Modus ponens is a valid form of argument. pq p p q pq p T T T T
T F F T F T T F F F T F q T q 60 Logical Form and Logical Equivalence Conditional Statements
Valid and Invalid Arguments Modus Ponens and Modus Tollens Modus tollens is another valid form of argument. If p then q ~q ~p 61 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Modus Ponens and Modus Tollens: Quick Quiz Use modus ponens or modus tollens to fill in the blanks of the following arguments so that they become valid inferences. a. If there are more pigeons than there are pigeonholes, then at least two pigeons roost in the same hole. There are more pigeons than there are pigeonholes. At least two pigeons roost in the same hole. _____________________________________ b. If 870,232 is divisible by 6, then it is divisible by 3. 870,232 is not divisible by 3. 870,232 is not divisible by 6. _____________________________________
62 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Additional Valid Argument Forms: Rules of Inference 2.3.4. Additional Valid Argument Forms: Rules of Inference A rule of inference is a form of argument that is valid. Thus modus ponens and modus tollens are both rules of inference. Other rules of inference: 1. 2. 3. 4. 5. Generalization Specialization Elimination Transitivity Proof by Division into Cases 63 Logical Form and Logical Equivalence
Conditional Statements Valid and Invalid Arguments Rules of Inference: Generalization 2.3.4.1. Rules of Inference: Generalization The following argument forms are valid. p q pq pq Example: Anton is a junior. (More generally) Anton is a junior or Anton is a senior. 64 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Rules of Inference: Specialization 2.3.4.2. Rules of Inference: Specialization The following argument forms are valid. Example:
pq pq p q Allows you to discard extraneous information to concentrate on the particular property of interest. Ana knows numerical analysis and Ana knows graph algorithms. (In particular) Ana knows graph algorithms. So if you are looking for someone who knows graph algorithms to work with you on a project, and you discover that Ana knows both numerical analysis and graph algorithms, would you invite her to work with you on your project? 65 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Rules of Inference: Elimination 2.3.4.3. Rules of Inference: Elimination The following argument forms are valid.
pq pq ~q ~p p q When you have two possibilities and you can rule one out, the other must be the case. Example: Suppose you know that for a particular number x, x 3 = 0 or x + 2 = 0 If you also know that x is not negative, then x -2, so by elimination you can conclude that x = 3. 66 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Rules of Inference: Transitivity 2.3.4.4. Rules of Inference: Transitivity
The following argument form is valid. pq Many arguments in mathematics contain chains of if-then statements. qr From the fact that one statement implies a second and the second implies the third, you can conclude that the first statement implies the third. pr Example: If 18,486 is divisible by 18, then 18,486 is divisible by 9. If 18,486 is divisible by 9, then the sum of the digits of 18,486 is divisible by 9. If 18,486 is divisible by 18, then the sum of the digits of 18,486 is divisible by 9. 67 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Rules of Inference: Proof by Division into Cases 2.3.4.5. Rules of Inference: Proof by Division into Cases The following argument form is valid.
pq pr qr It often happens that you know one thing or another is true. If you can show that in either case a certain conclusion follows, then this conclusion must also be true. r Example: Suppose you know that x is a nonzero real number. The trichotomy property of the real numbers says that any number is positive, negative, or zero. Thus (by elimination) you know that x is positive or negative. You can deduce that x2 > 0 by arguing as follows: x is positive or x is negative. If x is positive, then x2 > 0. If x is negative, then x2 > 0. x2 > 0. 68 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments
Rules of Inference: Example 2.3.4.6. Rules of Inference: Example You are about to leave for school in the morning and discover that you dont have your glasses. You know the following statements are true: a. b. c. d. e. If I was reading the newspaper in the kitchen, then my glasses are on the kitchen table. If my glasses are on the kitchen table, then I saw them at breakfast. I did not see my glasses at breakfast. I was reading the newspaper in the living room or I was reading the newspaper in the kitchen. If I was reading the newspaper in the living room then my glasses are on the coffee table. So, where are your glasses? 69 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Rules of Inference: Example
Let RK = I was reading the newspaper in the kitchen. GK = My glasses are on the kitchen table. SB = I saw my glasses at breakfast. RL = I was reading the newspaper in the living room. GC = My glasses are on the coffee table. Here is a sequence of steps you might use to reach the answer, together with the rules of inference that allow you to draw the conclusion of each step: 1. RK GK by (a) GK SB by (b) RK SB by transitivity 3. RL RK ~RK RL by (d) by conclusion of (2) 2. RK SB ~SB
~RK RK 4. RL GC RL GC by (e) by conclusion of (3) by conclusion of (1) by (c) by modus tollens by elimination by modus ponens Thus the glasses are on the coffee table. 70 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies 2.3.5. Fallacies A fallacy is an error in reasoning that results in an invalid argument. Three common fallacies:
1. Using ambiguous premises, and treating them as if they were unambiguous. 2. Circular reasoning (assuming what is to be proved without having derived it from the premises) 3. Jumping to a conclusion (without adequate grounds) 71 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies For an argument to be valid, every argument of the same form whose premises are all true must have a true conclusion. It follows that for an argument to be invalid means that there is an argument of that form whose premises are all true and whose conclusion is false. 72 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies: Converse Error
2.3.5.1. Fallacies: Converse Error Example: If Zeke is a cheater, then Zeke sits in the back row. Zeke sits in the back row. Zeke is a cheater. pq qp q q p p Converse error is also known as the fallacy of affirming the consequence. 73 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies: Inverse Error 2.3.5.2. Fallacies: Inverse Error Example: If interest rates are going up, stock market prices will go down. Interest rates are not going up.
Stock market prices will not go down. pq ~p ~RK q ~p ~p ~q ~q 74 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies: A Valid Argument with a False Premise and a False Conclusion 2.3.5.3. Fallacies: A Valid Argument with a False Premise and a False Conclusion The argument below is valid by modus ponens. But its major premise is false, and so is its conclusion. If Joseph Schooling is a Singaporean, then Joseph Schooling is a badminton player. Joseph Schooling is a Singaporean. Joseph Schooling is a badminton player.
75 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies: An valid Argument with True Premises and a True Conclusion 2.3.5.4. Fallacies: An Invalid Argument with True Premises and a True Conclusion The argument below is invalid by the converse error, but it has a true conclusion. If Singapore is a garden city, then Singapore has lots of trees. Singapore has lots of trees. Singapore is a garden city. 76 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Fallacies: Sound and Unsound Arguments 2.3.5.5. Fallacies: Sound and Unsound Arguments Definition 2.3.2 (Sound and Unsound Arguments) An argument is called sound if, and only if, it is valid and
all its premises are true. An argument that is not sound is called unsound. 77 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Contradictions and Valid Arguments 2.3.6. Contradictions and Valid Arguments The concept of logical contradiction can be used to make inferences through a technique of reasoning called the contradiction rule. Suppose p is some statement whose truth you wish to deduce. Contradiction Rule If you can show that the supposition that statement p is false leads logically to a contradiction, then you can conclude that p is true. 78 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Contradictions and Valid Arguments: Example Contradiction Rule Show that the following argument form is valid:
~p false p premise conclusion p ~p c ~p false p T F F T T F T F F
Only one critical row, and in this row the conclusion is true. Hence this form of argument is valid. 79 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Contradictions and Valid Arguments: Example Contradiction Rule The contradiction rule is the logical heart of the method of proof by contradiction. A slight variation also provides the basis for solving many logical puzzles by eliminating contradictory answers: If an assumption leads to a contradiction, then that assumption must be false. 80 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Summary of Rules of Inference
2.3.7. Summary of Rules of Inference Table 2.3.1 Rule of inference Modus Ponens pq p q Modus Tollens Generalization Specialization pq ~q ~p p pq q pq p Conjunction pq pq q p
q pq 81 Logical Form and Logical Equivalence Conditional Statements Valid and Invalid Arguments Summary of Rules of Inference 2.3.7. Summary of Rules of Inference Table 2.3.1 (contd) Rule of inference Elimination pq ~q p pq ~p q Transitivity pq qr pr Proof by Division
Into Cases pq pr qr r ~p false p Contradiction Rule 82 Next weeks lectures 3. The Logic of Quantified Statements 83 END OF FILE 84