Daniel Lemire's blog

, 8 min read

A little brain teaser…

12 thoughts on “A little brain teaser…”

  1. Parand says:

    Since the Falsian wouldn’t say they always disagree, Second must be the Truan, and therefore you should be worried.

  2. Jon says:


  3. Kevembuangga says:

    First Byp a liar Second Byp a liar

    Stmt 3 False True
    Stmt 4 True ****

    It cannot be that Stmt 4 by Second Byp be true if Falsians ALWAYS lie, therefore the First Byp is the Falsian => worried.

    What’s most interesting is that such a simple problem appear puzzling, there must be some step in our deductions which goes against our natural tendencies, r.e. Johnson Laird’s Mental Models.

  4. İsmail Arı says:


    Case 1, Both are Truans or both are Falsians: They can not meet with different greetings. Conflict!

    Case 2, FB is Truan and SB is Falsian: SB is telling the truth at the end of the conversation but he should’t. So this can not be the case.

    Case 3, FB is Falsian and SB is Truan: All statements seem convenient, so we should be worried.

    I do not remember but there was a similar puzzle where the person should find a single question that will allow him to identify which is Truan and which is Falsian.

  5. Mark Reid says:

    Nice puzzle and I came to the same conclusion as the other comments.

    Now you’re warmed up, you can try the Hardest Logic Puzzle Ever. (Watch out for the spoilers further down on the same page).

  6. Anthony says:

    Don’t worry, be happy !

    When asked : “Are you a Falsian ?”, they should both answer “No”. Therefore, the two Bypolarians don’t always disagree.
    Therefore, the second Bypolarians is a Falsian, and you should not worry.

  7. Anonymous says:

    To Anthony, you are puzzling us, but you are wrong. “Are you a Falsian ?”,this is actually two different questions.

  8. Paul Ogilvie says:

    Here’s my question: how could an explorer discover the Truthian and Falsian species and their properties in the first place? Without assuming that they always tell the truth or always lie, is there a series of questions an explorer could ask where the Truthian/Falsian nature of Bypolarians is the only valid conclusion? (I don’t know the answer to this question.)

  9. rg says:

    There’s one point that puzzles me: statement #4 does not necessarily need to be true. The negation of “we always disagree” is “we don’t always disagree”, not “we never disagree”. So it is plausible that Byp #2 is a Falsian.

    I go with Anthony. Although some may say the question “are you a Falsian” is another problem, it fits this problem well: both should answer “no” and we know here that they don’t always disagree. Hence Byp #2 must be the liar.

  10. Anonymous says:

    Worried – you’re on a planet where half the people will lie to you.

  11. Kevembuangga says:

    İsmail Arı I do not remember but there was a similar puzzle where the person should find a single question that will allow him to identify which is Truan and which is Falsian.

    The single question is “What would the other say if I asked him if he is a liar?”

    This is easy because it is only a matter of logic, but I suspect that Daniel intent is more mischievous, to highlight the ambiguity of natural language use for stating logic/math problems.
    Because everything depends on how you define “always disagree” and this cannot be elucidated from the problem statement alone.

  12. Anonymous says:

    “Are you a Falsian?”, even though both of them answer “no”, they are actually answering different questions. So it’s not a surprise they give the same answer.
    A is answering “Is A a Falsian?”
    B is answering “Is B a Falsian?”
    Of course they give the same answer “no”.