CSCE 235

Handout 8: Monty Python and the Holy Grail

January 30, 2003 

 

The following example in Predicate Logic is based on the movie “Monty Python and the Holy Grail” (1975).  Here is an excerpt of the script for scene #5:

 


Scene 5

 

VILLAGER #1:  We have found a witch.  May we burn her?

CROWD:  Burn her!  Burn!  Burn her!  Burn her!

BEDEVERE:  How do you know she is a witch?

VILLAGER #2:  She looks like one.

CROWD:  Right!  Yeah!  Yeah!

BEDEVERE:  Bring her forward.

WITCH:  I'm not a witch.  I'm not a witch.

BEDEVERE:  What makes you think she is a witch?

VILLAGER #3:  Well, she turned me into a newt.

BEDEVERE:  A newt?

VILLAGER #3:  I got better.

VILLAGER #2:  Burn her anyway!

VILLAGER #1:  Burn!

CROWD:  Burn her!  Burn!  Burn her!...

BEDEVERE:  Quiet!  Quiet!  Quiet!  Quiet!  There are ways of telling whether she is a witch.

VILLAGER #1:  Are there?

VILLAGER #2:  Ah?

VILLAGER #1:  What are they?

CROWD:  Tell us!  Tell us!...

BEDEVERE:  Tell me.  What do you do with witches?

VILLAGER #2:  Burn!

VILLAGER #1:  Burn!

CROWD:  Burn!  Burn them up!  Burn!...

BEDEVERE:  And what do you burn apart from witches?

VILLAGER #1:  More witches!

VILLAGER #3:  Shh!

VILLAGER #2:  Wood!

BEDEVERE:  So, why do witches burn?

    [pause]

VILLAGER #3:  B--... 'cause they're made of... wood?

BEDEVERE:  Good!  Heh heh.

CROWD:  Oh, yeah.  Oh.

BEDEVERE:  So, how do we tell whether she is made of wood?

VILLAGER #1:  Build a bridge out of her.

BEDEVERE:  Ah, but can you not also make bridges out of stone?

VILLAGER #1:  Oh, yeah.

RANDOM:  Oh, yeah.  True.  Uhh...

BEDEVERE:  Does wood sink in water?

VILLAGER #1:  No.  No.

VILLAGER #2:  No, it floats!  It floats!

VILLAGER #1:  Throw her into the pond!

CROWD:  The pond!  Throw her into the pond!

BEDEVERE:  What also floats in water?

ARTHUR:  A duck!

CROWD:  Oooh.

BEDEVERE:  Exactly.  So, logically...

VILLAGER #1:  If... she... weighs... the same as a duck,... she's made of wood.

BEDEVERE:  And therefore?

VILLAGER #2:  A witch!

VILLAGER #1:  A witch!

CROWD:  A witch!  A witch!...

VILLAGER #4:  Here is a duck.  Use this duck.

    [quack quack quack]

BEDEVERE:  Very good.  We shall use my largest scales.

CROWD:  Ohh!  Ohh!  Burn the witch!  Burn the witch!  Burn her!  Burn her!  Burn her!  Burn her!  Burn her!  Burn her!  Burn her!  Ahh!  Ahh...

BEDEVERE:  Right.  Remove the supports!

    [whop]

    [clunk]

    [creak]

CROWD:  A witch!  A witch!  A witch!

WITCH:  It's a fair cop.

VILLAGER #3:  Burn her!

CROWD:  Burn her!  Burn her!  Burn her!  Burn!  Burn!...

BEDEVERE:  Who are you who are so wise in the ways of science?

ARTHUR:  I am Arthur, King of the Britons.

BEDEVERE:  My liege!

ARTHUR:  Good Sir Knight, will you come with me to Camelot and join us at the Round Table?

BEDEVERE:  My liege!  I would be honored.

ARTHUR:  What is your name?

BEDEVERE:  'Bedevere', my liege.

ARTHUR:  Then I dub you 'Sir Bedevere, Knight of the Round Table'.


 

Here is the logic:

 

1.                                                     Rule 1 (Hypothesis)

2.                                  Rule 2 (Hypothesis)

3.                                      Rule 3 (Hypothesis)

4.                                                                                      Fact 1  (Hypothesis)

5.                                                                                     Fact 2  (Hypothesis)

6.                                                                                      Fact 3  (Hypothesis)

7.                                                                                     Fact 4  (Hypothesis)

 

(Note:  Polly is the character name of the actress, who played the witch, in another TV show)

Experiment:  ?  Result:   is true.

 

8.                                                                     Fact 5 (Hypothesis)

9.                              3; Universal Instantiation

10.                     9; Universal Instantiation

11.                                              8, 4; conjunction

12.                                                                                    10, 11; modus ponens

13.                        2; Universal Instantiation

14.                 13; Universal Instantiation

15.                                                              6, 5; conjunction

16.                                         15, 12; conjunction

17.                                                                                    14, 16; modus ponens

18.                                       1; Universal Instantiation

19.                                                              17, 7;  conjunction

20.                                                                                     19, 18; modus ponens

 

So, we have proved, given the hypotheses, that Polly is a witch.  So, it is actually logically correct.