I always lie. Do you think I'm lying?

if you always lie, then you are being truthful, so you must be lying.

on the other hand, since you are being truthful about the fact that you are lying, you aren't lying.

since you aren't lying, you are telling the truth, so you must be lying, therefore...(repeat as needed)

your sentence is a self-negation, without any possible assignable truth-value.

determining whether or not it's true, depends on assigning a truth value to "i always lie", which in turn requires assigning the opposite truth value to "i always lie" making it impossible to determine whether of not we (or i) think you're lying.

it's like this: p<-->(¬p), no matter whether we assign p = true, or p = false, we reach a contradiction.

in short, your sentence is not meaningful, it is similar to saying: i am only happy if i am sad. this points out that the law of the excluded middle has its short-comings, not every sentence MUST be either true or false, some sentences can be neither.

Its a paradoxical statement. If you truly do always lie, then this statement is true, meaning you don't always lie. So you cannot be telling the truth when you say you always lie, thus you are lying when you say you always lie.

Hm, strange that the others didn't think this is a logical question.

Yes you are.

If you really would be lying always, you couldn't say this, because then you would speak the truth, and thus you weren't always lying.

Apparently you are not always lying. So whenever you say you are, you are lying.

Conclusion: you are lying right now!

why don't ask this question in the xmas section?

yes :D