what's a tautology

[Dak]

I've looked it up from many sources, and even searched this forum to see how the word was used, and i plain and simple don't really understand what it is, and specifically why it's bad.

[CharonY]

Tautologies are in general some kind of truisms or repetitions that possess some kind of redundancy and hence do not add information.

A tautological logical statemen is for instance: "all swans are white or not."

This statement will always be true making this an empty statement.

In everyday language tautologies can be empty repetitions like e.g."Salsa sauce" or "HTML-language."

In most cases it is considered a bad style because you essentially do not add any information but only repeat yourself by stating the same (this is another one btw).

[Dak]

I see.

 

But i've heard that, whilst they're not faliciouse, they shouldn't be used in logical arguments (not just because they're redundant). any idea why?

[bascule]

It's not just that they're redundant, it's that they're always true because the conclusion and premise are the same

[AL]

But i've heard that, whilst they're not faliciouse, they shouldn't be used in logical arguments (not just because they're redundant). any idea why?

A tautology is a statement that is trivially true because it contains all logical possibilities, like CharonY illustrated: "All swans are white or not white." The reason it isn't helpful to make an argument from it is because it's vacuous. The statement is true, but clearly, it has told us nothing about the color of swans.

 

Some more examples:

 

"You are either a girl or not a girl" is true, but doesn't tell us what your gender is.

 

"You either drive a car, or you do not" is true, but doesn't tell us if you drive a car.

 

And so on.

 

"This statement is either true or false" is true...or is it?

[Dak]

ok. i understand what a tautology is now (ta).

 

but how could it be used as a rebuttal. for example, i've heard it said (more or less) that 'that's invalid because it's tautological'.

 

I would have thought that, being true by necessity, you couldnt say that a tautology is invalid?

[CharonY]

Bascule,

you are of course correct. What I meant with repetition is a tautaology as a stylistic device rather than in the context of logical statements. I should have been more preciese.

 

Dak,

the statement is true, but tautologies are invalid as a logical argument. Basically in the same way that circular arguments are.