Some puzzles about truth
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Some Puzzles About Truth. (and belief, knowledge, opinion, and lying). Puzzle #1: the postcard paradox. Consider the following sentences: 1) the following sentence is true: 2) the preceding sentence is false. (examine the index card now circulating) Q: are these sentences true or false?.

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Some Puzzles About Truth

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Some puzzles about truth

Some Puzzles About Truth

(and belief, knowledge, opinion, and lying)


Puzzle 1 the postcard paradox

Puzzle #1: the postcard paradox

  • Consider the following sentences:

    1) the following sentence is true:

    2) the preceding sentence is false.

  • (examine the index card now circulating)

  • Q: are these sentences true or false?


Puzzle 2 the liar paradox

Puzzle #2: the liar paradox

  • Is the following proposition true or false?

    This proposition is false

  • • If every proposition is either true or false then this proposition will be either true or false

  • • If it is true, then it is true that it is false; so it must be both true and false

  • • If it is false, then it is false that it is false; so it must be true; so it must be both true and false

  • • So in both cases it is both true and false, which is impossible


Puzzle 3 on the island of knights and knaves

Puzzle #3: On the island of knights and knaves

  • On the island of Knights and Knaves, every inhabitant is either a knight or a knave. Knights always tell the truth. Knaves never tell the truth; any sentence uttered by a knave is false. A stranger came to the island and encountered three inhabitants, A, B, and C. He asked A, "Are you a knight, or a knave?" A mumbled an answer that the stranger could not understand. The stranger then asked B, "What did he say?" B replied, "A said that there is exactly one knight among us." Then C burst out, "Don't believe B, he is lying!" What are B and C?

  • One day I went to the island of knights and knaves and encountered an inhabitant who said, "Either I am a knave or else two plus two equals five." What should you conclude?


Puzzle 4 the well named ill named paradox

Puzzle #4: the ‘well-named’ ‘ill-named’ paradox

  • Have you ever noticed that some people are very well named? Martin Short is, after all, rather short. I once met a realtor named 'Isolde Haus' and a preacher named 'Mike Pentacost'. Just recently, I received a letter from an evolutionary biologist named 'Steve Darwin'.

  • Let's call everyone else 'ill-named'. Some people who are ill-named are rather spectacularly ill-named. For example, Tiny Tim is really rather large. Most of us are ill-named in a less interesting way, though. In any case, let's just agree to call everyone who isn't well-named 'ill-named'.

  • I used to play a game of classifying everyone I met as well named if their name is, somehow, particularly appropriate for them and ill named if it is not. I quit playing this game when a new neighbor moved in next door. His name is John Ill-named. Is he well named, or ill named? (due to Raymond Smullyan)


Puzzle 5 how to prove anything

Puzzle #5: how to prove anything

  • Let A be any arbitrary sentence, and let B be the sentence "If this sentence is true, then A is true". Suppose B is, in fact, true. Then, according to B, A is true. Thus, we have established that if B is true, then A is true. But this is exactly what B asserts! Thus, B must be true, from which it follows (by B) that A must be true. Hence, all sentences are true! (due to M. H. Lob)


Thinking about the puzzles

Thinking about the puzzles


Puzzle 6 the lottery paradox

Puzzle #6: the lottery paradox

  • Imagine a fair lottery with a thousand tickets in it. 

  • Each ticket is so unlikely to win that we are justified in believing that it will lose. 

  • So we can infer that no ticket will win. 

  • Yet we know that some ticket will win.


Puzzle 7 the preface paradox

Puzzle #7: the preface paradox

  • Authors are justified in believing everything in their books. 

  • Some preface their book by claiming that, given human frailty, they are sure that errors remain, errors for which they take complete responsibility. 

  • But then they justifiably believe both that everything in the book is true, and that something in it is false.

  • Q: does this paradox look familiar? 


Puzzle 8 the knowability paradox

Puzzle #8: the knowability paradox

  • The following two claims seem eminently rasonable: a) some truths are not known, and b) any truth is knowable. 

  • Since the first claim is a truth, it must be knowable. 

  • From these claims it follows that it is possible that there is some particular truth that is known to be true and known not to be true.


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