Here is another question I didn't get a clear answer to when the term "consistency" is applied in philosophy, logic and computer science.

Is always lying consistent since we know the statements are false? For example please assume that some person or system always makes false statements. Knowing this I could deduce everything from false statement e.g. getting the false statement that bird can't fly I would conclude that since the system or person is always lying then I just know the negation of that information is the truth.

But isn't it contradictory that a system that always tells me a lie is a consistent system? I can deduce truth and evidence from the system knowing that it is always false, so is it my interpretation that makes the notion of consistence combining the knowledge that the statements are false and therefore making false statement is indeed inconsistent and it's only applying the logic that the statements are false that makes the notion of consistency when the statement are true?

Thank you for any comments and/or answers

3 Answers 3


It is indeed consistent.

Consistency in rough words could finally sum upto doing stuff repetetively. Consistence has no degree of accuracy or the degree of correctness of the phenomenon said to be consistent.

What you say is negative consistency but its still consistent provided that the negative statements never change to be positive ideally.

Hence to sum up consistency is something that occurs in a repetitive pattern with no degree of accuracy or correctness of its outcome.

  • thank you for developing the topic. I've been suspecting that answering whether a set of statements are consistent is not enough information whether the statements are true or false. Being consistent seems to be more like being deterministic and "accordingly" as long as you don't break the rules and just being consistent doesn't necessarily mean being a preferred choice. Aug 17, 2011 at 14:19
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    its a really good topic actually, there are quite a few stuff like this that occasionally pops up here and there :)
    – swordfish
    Aug 17, 2011 at 15:37

I think you might want to start your research here.

It appears that you are confusing the philosophical notion of a "consistent system" (meaning a system devoid of contradictions) with the everyday notion of "consistency" (as in "behaving in a consistent, predictable way.")

Saying that "Always lying about everything is a consistency" is either a truism or a category error, depending on which sense you are intending.

Taking your premise of a person who always lies: Given a system that is consistent (let's say, a small number of undisputed facts about the world) and a person who always lies, the set of statements uttered by this person will not be a consistent system. Let's say the sky is blue. Our Liar can then say "The sky is orange" and "The sky is green", which results in a contradiction.

(Note that you are incorrect in saying that you can deduce everything from the false statements, since the negation of the false statements is the truth: in the example given, you can deduce that the sky is neither green nor orange, but you do not have enough information to deduce that it is blue.)

As I said in an answer to a related question of yours, I think you might want to sit down with a good introductory undergraduate logic textbook.

  • thank you Michael it was a very good and methodological example with the Sky being orange and green resulting in a contradiction. I hadn't though of this and instead used terms consistent and unbiased about statistical estimators where an estimate that always is 2 % wrong would have consistency but not unbias(?) Aug 17, 2011 at 14:22
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    "Consistency" in statistics means something different than "consistency" in logic. Most of the real work in philosophy is in adequately defining terms-- once that is done, the rest is usually easy. Aug 17, 2011 at 18:51

I would say no. For example:

  • All fish are green.
  • All fish are red.
  • All fish are blue.

Those statements would hardly be considered consistent, but they are all lies. The point I'm making is that for most true statements there are many distinct negations of that statement. One can pick and choose among the lies to be as consistent or inconsistent as you wish.

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