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Valid arguments A valid argument has the following property: It is impossible for the conclusion to be false, if we assume that every premise is true.

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Presentation on theme: "Valid arguments A valid argument has the following property: It is impossible for the conclusion to be false, if we assume that every premise is true."— Presentation transcript:

1 Valid arguments A valid argument has the following property: It is impossible for the conclusion to be false, if we assume that every premise is true. In a valid argument, the truth of the premises forces the truth of the conclusion.

2 Use of truth tables to analyze arguments 1. Symbolize (consistently) all of the premises and the conclusion. 2. Make a truth table having a column for each premise and for the conclusion. 3. If there is a row in the truth table where every premise column is true but the conclusion column is false (a counterexample row) then the argument is invalid. If there are no counterexample rows, then the argument is valid.

3 Why does this method work? When we have filled in the truth table, we are checking to see if there is a row where the conclusion is false while every premise is true. If there is a row where the conclusion is false while every premise is true, then the truth table has shown that it is possible for the conclusion to be false at the same time that every premise is assumed to be true: this is exactly the definition of an invalid argument.

4 Exercise Use a truth table to test the validity of the following argument. If I enter the poodle den, then I will carry my electric poodle prod or my can of mace. I am carrying my electric poodle prod but not my can of mace. Therefore, I will enter the poodle den. A. Valid B. Invalid

5 Solution If I enter the poodle den, then I will carry my electric poodle prod or my can of mace. I am carrying my electric poodle prod but not my can of mace. Therefore, I will enter the poodle den. Step 1: Symbolize Let p: I enter the poodle den q: I carry my electric poodle prod r: I carry my can of mace p (q r) q ~r p

6 Solution, page 2 Step 2: Make a truth table having a column for each premise and a column for the conclusion. p (q r) q ~r p

7 Solution, page 3 Step 3: Is there a row where the conclusion is false while both premises are true? Yes, the sixth row shows that the argument is INVALID.

8 Exercise Test the validity of the argument. I dont like muskrats. If I own a badger or I dont own a wolverine, then I like muskrats. Therefore, I own a wolverine and I dont own a badger. A. Valid B. Invalid

9 Solution I dont like muskrats. If I own a badger or I dont own a wolverine, then I like muskrats. Therefore, I own a wolverine and I dont own a badger. First, reduce the argument to symbols. Let p: I like muskrats q: I own a badger r: I own a wolverine With this choice of symbols, the argument as the following form: ~p (q ~r) p r ~q

10 Solution, page 2 Step 3: There is NO ROW where the conclusion is false while both premises are true, so the argument is VALID. Step 2: Make a truth table having a column for each premise and a column for the conclusion. ~p (q ~r) p r ~q

11 Exercise The Argue-mentor, Part 3 Test the validity of this argument. ~p ~q q ~p A.Valid B. Invalid

12 Solution Below is a truth table for the argument. (There may be some redundant columns.) The three columns in bold are the important columns. An argument is INVALID if it is possible for the conclusion to be false while every premise is true. There is no row where the conclusion is false while every premise is true, so the argument is valid.


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