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If a guy says "I'm a liar", what could we deduce from it? Is he a liar or not?

let us say he isn't a liar, then his statement that he's a liar affirms that he is a liar.

Let us say he is a liar then his statement that he is a liar again affirms that yes he's a liar which deduces that he is telling the truth and isn't a liar.

If he is a liar then he is lying at being a liar

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  • $\begingroup$ a liar not always is a liar $\endgroup$ – lois6b Sep 15 '16 at 10:43
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    $\begingroup$ this should be the most ridiculous question i have ever seen... $\endgroup$ – Oray Sep 15 '16 at 10:48
  • $\begingroup$ We can deduce that we cannot deduce. $\endgroup$ – Ivanhoe Sep 15 '16 at 10:51
  • $\begingroup$ I'm still thinking about the reason :D $\endgroup$ – Ahmad Sep 15 '16 at 10:54
  • $\begingroup$ Assuming the person is a perfect truth teller or perfect liar, he or she will never say "I am a liar" for the reasons you state above (that the statement is a contradiction). If someone does say that, you can either say that you conclude this person is not perfect or the question is broken. $\endgroup$ – mmking Sep 15 '16 at 14:10
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According to a (now deleted) the comment, this indiviual is "born to lie". If this means that he will always lie, then he wouldn't say "I'm a liar" because that would mean he just told the truth.

This is called the Liar paradox.

If we don't know if the person is a liar

If we don't know whether the person is a liar or not, we can at least deduce that he doesn't always say the truth. That is because if his statement is true, it means he has lied before. If his statement is false, it creates another paradox or it at least means that what he has just told is a lie.

The types of liars

Because OP is confused about what a liar is, I'll add that to my answer as well.

  • Logical liar: The type of liar found in logic puzzles. He always lies. This is a fictional entity, as it would create a paradox, would he ever say things like "I am lying".
  • Real life liar: A person who uses lies to their advantage, lies a lot and likes to lie.
  • Liar per definition: Someone who tells lies. You can be called a liar even if you lied only 1 time ever.

Edit: There's some new information in the answer, so let's handle it:

let us say he isn't a liar, then his statement that he's a liar affirms that he is a liar.

If he isn't a liar, his statement is false, making him a liar. That, in turn, makes his statement true. Because it's not true, he isn't a liar anymore and therefore his statement is false again. From a logical standpoint, this is a paradox.

Let us say he is a liar then his statement that he is a liar again affirms that yes he's a liar which deduces that he is telling the truth and isn't a liar.

If he is a liar and says so then he is telling the truth with his statement. Depending on what type of liar he is, this either generates a paradox (logical liar) or is completely valid (normal liar).

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  • $\begingroup$ img.ifcdn.com/images/… $\endgroup$ – lois6b Sep 15 '16 at 10:57
  • $\begingroup$ what if we don't know if he's a liar? $\endgroup$ – Ahmad Sep 15 '16 at 11:01
  • $\begingroup$ @Ahmad Depends on if "being a liar" defines as "always lying" or "often/sometimes lying" $\endgroup$ – Manuel Hoffmann Sep 15 '16 at 11:05
  • $\begingroup$ why being lying has two meaning to you guys? A liar ever lies. No? $\endgroup$ – Ahmad Sep 15 '16 at 11:07
  • $\begingroup$ @Ahmad you deleted the comment where you assume he always lies. if he doesnt lies always, then he can say that affirmation without problems $\endgroup$ – lois6b Sep 15 '16 at 11:09

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