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:
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.