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Summary Notes

These are summary notes so that you can really listen in class and not spend the entire time copying notes. These notes will not substitute for reading the chapters in the text, nor can they replace the text as there are many subtleties we will discuss in class that are also presented in the text. Use these as a supplement.

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Section 6.1| Section 6.2 | Section 6.3 | Section 6.4 |

| Section 6.5 | Section 6.6 | Smartboard notes


Section 6.1 Symbols and Translation

Terms to learn:

The Operator Chart

Operator
Typed on Web
Name
Logical Function
Used to Translate

~

~ tilde negation not, it is false that, it is not the case that

·

&& dot conjunction and, also, but, moreover, however, nevertheless, still, both, additionally, furthermore 

V

\/ wedge disjunction or, unless

conditional

=> or /\ horseshoe implication if...then..., only if, given that, provided that, in case, on condition, that, sufficient condition for, necessary condition for

bi-conditional

<=> triple bar equivalence if and only if, is a necessary and sufficient condition for


Tips for translation:

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6.2 Truth Functions

 

 
 

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6.3 Truth Tables for Propositions

Templates: 4-line, 8-line, & 16-line truth tables

What are truth tables and why do we use them in logic?

What is the process for setting up a truth table?

  1. To calculate the number of lines in a truth table, use this simple formula: L =2n. L is the number of lines and the small "n" is a variable representing the number of simple propositions contained in the statement. Simple propositions are expressions represented by a single letter.

  2. Symbolize the statement you are evaluating.

  3. Determine the number of lines necessary using the formula presented above.

  4. Divide the total number of lines in half and assign the truth value "T" to the lines beneath the first simple proposition and assign the value "F" to the remaining lines.

  5. For each successive simple proposition, divide the number of "T" values in half again and assign the value "T"  to the first half and "F" to the second half.  This process sounds far more complicated than it actually is. To see the process in action follow through the sample arguments that follow.

  6. After truth values have been assigned to each simple proposition, evaluate the proposition working from inside the parenthesis to the outer layers.

  7. When you evaluate the column under the main operator, you will be asked to classify your argument in accordance with the categories below.

 

Statement Classification
Column Under Main Operator
tautologous (logically true) all true
self-contradictory (logically false) all false 
contingent at least one true, at least one false


Relation
Column Under Main Operators
logically equivalent same truth value on each line
contradictory opposite truth value on each line 
consistent there is at least one line on which the truth values are both true
inconsistent there is no line on which the truth values are both true



See Hurley's explanation  covering the classification of propositions. In short, we first try to classify a proposition as either logically equivalent of contradictory and then, if it does not meet that criteria, the categories of consistent and inconsistent apply.

Real World Applications:

Arguments that are logically consistent can be said to "make sense" because "there will be at least one line in the group of truth tables where all of the person's statements are true." Arguments that are contradictory or inconsistent indicate that there is not an instance where both the premises and conclusion are true.

 

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6.4 Truth Tables for Arguments

The process for testing the validity of arguments with truth table is detailed above. The process is the same is that for testing individual propositions, except we are now testing multiple propositions strung together. Thus, we will be looking for lines on which all premises  are true and the conclusion false signaling the argument is invalid. If no such line exists, then the argument is valid.

 

 

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6.5 Indirect Truth Tables

Indirect truth tables provide a shortcut method for testing argument validity. This exercise is a review of the skills we learned in Section 1.5 where the method of counterexample was introduced. In order to set up this shortcut method, we have to consider all possible situations in which all premises could be true and the conclusion false.

Testing arguments for validity:

The process for testing arguments for validity is described in the section. The method is:

  1. Look at the conclusion and determine the ways in which the main operator could evaluate "false." 

  2. Assign truth values of false to the conclusion and true for the premises for each situation in which the main operator in the conclusion could evaluate false.

  3. Working backwards from the truth values generated by evaluating the conclusion false, deduce the truth values for the remaining simple propositions and premises. 

  4. If you do not encounter a contradiction while assigning truth values to the premises, then it is possible for all premises to be true and the conclusion false. Thus, the argument is invalid.

  5. If a contradiction is produced in the attempts to assign truth values to the premises, circle the contradiction and declare the argument valid.

  6. For arguments with multiple ways of evaluating false:

    • you must generate a contradiction on every line for the argument to be valid.

    • You can stop evaluating on a single line when you produce a contradiction.

    • If you fail to produce a contradiction on any line, (i.e., your deductions produce true premises and a false conclusion),  you can stop evaluating the argument. It is invalid.

Depending on the order of assignment of truth values in the premises, contradictions can be produced in multiple ways if an argument is valid. So it is possible that the solution presented in the text will be different from the one that you deduce. As long as you have evaluated the simple and complex propositions correctly, your solution is equally valid.

Testing statements for consistency:

The process for testing statements for consistency is similar to testing arguments for validity. The process differs from the above in the following ways:

  1. Assign the value of true to the main operator of each statement.

  2. Pick one of the statements and create a line for each situation in which the main operator could evaluate true. 

  3. Deduce the remaining truth values on the line. 

  4. If no contradiction is produced, the truth values are consistent.

  5. If a contradiction is produced, the statements are inconsistent.

 

 

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6.6 Argument Forms and Fallacies

This section introduces common valid and invalid argument forms. Six valid and two invalid forms are introduced.  It is possible to affirm validity/invalidity for any of these forms and I invite you examine each one of them via the truth table method. Without further delay, here are the forms:

Disjunctive syllogism (DS): 

This argument form relies on the "method of elimination" for its validity. Note that one of the two disjuncts must be eliminated for this argument form to be valid. This is the case because "inclusive disjunction includes the possibility of both disjuncts being true."

p V q

~p         

q

 

Pure Hypothetical  Syllogism (HS): 

A hypothetical syllogism is a chain of conditional statements such that the consequent of one premise is the antecedent of the remaining premise. Not every argument with three conditional statements is a hypothetical syllogism.

      p => q

    q => r

p => r

Modus Ponens (MP): 

Modus ponens is dubbed "asserting mode" because it affirms the antecedent in the second premise and the consequent in the conclusion.

      p=> q

      p          

q

 

 Modus Tollens (MT): 

Modus tollens is dubbed "denying mode" because it denies the consequent in the second premise and the antecedent in the conclusion. See Hurley's explanation for the logic behind modus tollens.

      p => q

      ~q         

~p

 

Be sure not to confuse the invalid forms of  affirming the consequent and denying the antecedent with MP and MT. Although they look like their valid cousins, these latter two forms are invalid and can be proven so by using truth tables or counterexamples.

Constructive Dilemma (CD): 

Be sure to study this form closely as many of the exercises in the text and on our exams will mimic this form, but not match it. Hence it is wise to note that the second premise contains the antecedents of each conditional statement and the conclusion contains each consequent.

(p => q) · (r => s)

       p V r               

q  V s

 

Destructive Dilemma (DD): 

Again, a caution to study this form closely as many of the exercises in the text and on our exams will mimic this form, but not match it. Hence it is wise to note that the second premise denies the consequents of each conditional statement and the conclusion denies the antecedents.

(p => q) & (r  => s)

      ~q  V ~s               

~p V ~r

 

Refuting Constructive and Destructive Dilemmas:

There are two direct methods for refuting CD and DD arguments. The first is labeled "grasping by the horns" and it requires us to prove that one of the two conditional statements in the first conjunctive premise is false. Remember conditional statements evaluate false if the antecedent is true and the consequent is false.  The second technique is labeled "escaping between the horns" and in order to use it to prove invalidity, we must prove the disjunctive premise false.

An indirect method of refuting CD an DD arguments is to show that the arguer has not considered all possible scenarios in the conditional statements that comprise the conjunctive premise; depending on which of the two arguments you are attempting to refute, the disjunctive premise should remain unchanged and the remaining set of antecedents or consequents should be changed.

A method for recognizing argument forms:

 These tips are taken from the last few pages of section 6.6.

  1. Symbolize the ordinary language argument.

  2. Compare the valid argument forms to the argument you have symbolized and look for patterns that would signal you have a valid form.

  3. Remember, the statement p V q is logically equivalent to q V p and you may have to change the order of the disjunctive propositions to see the pattern.

  4. Negated letters may also be substituted for p, q, r, and s.

  5. The statement p is logically equivalent to the statement ~~p.

  6. The order of premises does not affect the form of an argument. You may have to rearrange the premises to recognize the form.

 

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Smartboard Notes from Chapter 6 Lectures:

Section 6.1:

Part 1 and 2 problems done in class:

Part 1: Homework

 

 

Section 6.2:

  Text Example: â

6.2 Part 1 Problems:

Homework from Section 6.2:

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Smartboard Notes for Sections 6.3 & 6.4:

 

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Homework Review Week 12 Sections 6.3 & 6.4

 

 

Smartboard Notes from Sections 6.5 & 6.6

 

 

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