Reductio Ad Absurdum

Anders and Bartholomew are on a train.

Anders: I think this is the same car we had on the way up.

Bartholomew: I don't think so...It's emptier, for one.

A: No, I know it is.

B: How do you know?

A: Your initials are carved on the seat.

(B inspects)

B: Huh! I don't remember carving anything.

A: You didn't.  I carved them there.

B: Why would you carve my initials on a seat?

A: So I wouldn't get blamed when they found them.

B: I don't remember that either.  Maybe I wasn't with you.

A: But you must have been.

B: How do you know?

A: Your initials are carved on the seat.

(B inspects)

B: Huh! I don't remember carving anything.

A: I don't think you've been eating enough carrots.

B: Enough for what?

A: Enough to improve your memory.

B: Carrots don't improve your memory; carrots give you night vision.
   Spinach improves your memory.  It's been scientifically proven.

A: You're full of shit.

B: Prove it.

(A contemplates.)

A: Alright, if spinach improves your memory so much, they'd feed it to
   all the kids at boarding school, right?  My brother went to
   boarding school for two years and he never once mentioned spinach.
   QED.  Reductio ad absurdum.

B: Re-duck what?

A: Reductio ad absurdum.  Assume a propostion is true.  Make only
   logical deductions.  If you can prove something absurd, then your
   original proposition was false.

B: What if I prove something absurd without assuming anything?  Have I
   proven the entire world false?

A: Now you're being silly--

B: Or absurd?

A: Silly.

Pause.

B: Perhaps he forgot.

A: Who forgot?

B: Your brother.  

A stares blankly.

B: Perhaps your brother forgot to mention how much spinach he ate.

A: Impossible.  My brother ate two carrots a day.  He could never
   forget!

(Pause)

B: Perhaps you forgot.

A: Forgot what?

B: Perhaps your brother did tell you that he eats spinach everyday,
   but you forgot. 

A: Inconceivable!  I eat three carrots every day!

B: You eat three carrots a day?

A: Sometimes more.

B: And yet you never mentioned it to me, your best friend?

A: Must have slipped the mind...

(Pause)

B: Your brother never went to boarding school!

A: Are you calling me a liar?

B: Did your brother every go to boarding school?

A: You think I'm a liar.

B: Did your brother ever go to boarding school?

A: Just because my brother never went to boarding school doesn't make
   me a liar.  A liar is one who habitually and repeatedly tells
   untruths.

B: Untruths?

A: Things that aren't strictly true.

B: Like if you told me your brother went to boarding school when, in
   point of fact, you have no brother.

A: Exactly.

B: You have no brother.

A: No, but that doesn't mean I'm a liar.

B: What's a liar?

A: I don't know.  I forget.  

B: I don't think you've been eating enough carrots.