Nous devons utiliser une tierce personne comme témoin. (We must use a third party as a witness.)

Question

This example contradicts the rule immediate preceding it:

You use tiers (masculine) or tierce (feminine) instead of troisième when expressing a fraction, a portion of something (= one third of), as opposed to a rank or an order (= third out of).

A third party follows the some nomenclature as the third person. It is not a third of a party as if someone was having their legs chopped off. A third party to an agreement is anyone who isn't one of the signatories (ie., 'you and me'.)

It seems therefore to be an exception to the rule stated, rather than an example of the rule, as it is presented.

addendum;

This also appears to apply to "third-world", as there is a first world (NATO/OTAN countries), second world (communist bloc countries), and third-world (non-aligned countries).

The rule given in this lesson also seems to conflict with Cécile's comment on https://french.kwiziq.com/questions/view/in-the-example-j-ai-bu-un-tiers-de-la-bouteille-the-audio-speaker-is-a-woman-and-the-word-bottle-is where she says: "The adjective 'tiers/tierce' will only be used in certain expressions...It has very limited use...'.

I interpret this as 'the adjective "tiers/teirce" is only used in special cases, which you will just have to learn'.

Accepted Answer

Bonjour all,
This lesson has now been reviewed and updated. 
I hope this is helpful.
Bonne journée !

Answer

Neither '3rd person' (witness) nor '3rd world' are rank order descriptions. They are both indeed part of a specific number of 3 parts to the whole (fractions even) - 3 parties needed to validate a document, and 3 world 'divisions' commonly used. There is not meant to be any suggestion that one or other part has primacy, nor that there are any other parts. The descriptions as given in the lesson fit this well.

Answer

There isn't three parts to the whole: the definition of "third party" here is "a person other than the principals".

In everyday life, the term is more frequently encountered when buying insurance, and it is typically the bare minimum cover allowed by law. It is where the insured (first party) buys liability coverage from an insurer (second party) against claims by another (third party).

These parties are very clearly ordered. Also, notice how, from the perspective of the insured, this exactly follows "first, second and third person" in grammar...where third person is «troisième personne».

As to "3 parties required to validate a document" this fundamentally misunderstands the role of a third-party witness. The only role the witness would play if one of the principals was to claim that they didn't sign the document. In this case, the witness could be called to testify that they saw the document being signed. Only one person is validating the document here. This is, again, a very different role and much lesser involvement. So, again, this seems very clearly ranked.

The presence of an overarching group that they can be placed in simply doesn't lead to the use of 'tiers/tierce'. For example, the third film in a trilogy is the «le troisième film». Even when we're talking about a very clear subdivision, like the "period of life when professional activities cease" we say «troisième âge». Nor does use of tiers even mean something is divided into parts! For example, a three year old wild boar is a «tiers-an» (also «tiers-ans»).

I don't see this 'rule' as useful at all. Even after ignoring all the exceptions, it seems that to use it you have to put the cart before the horse. That is you must first know what the answer should be and work your way back from there. Even with the benefit of knowing the answer for "third party" seems to have involved changing the definition of it.

I can only imagine the contortions that will be required to explain why a «tiers-an» means a «sanglier de trois ans» and not a «sanglier de quatre mois».

(And even if someone manages that, if there's a rule, why has a Kwiziq staff member has provided a different explanation?)

Answer

Hi Céline,
I've reviewed the updated lesson, and I still believe that how the lesson is presenting the rule is wrong. When "tiers" is an adjective, the rule stated isn't helpful, yet the exercise gives examples as though it's consistent and logical.
First World? Premier Monde.
Third World? Tiers Monde.
Third part (portion of a document)?Troisième Partie. 
Third person (legal)? Tierce Partie. 
Third person (grammar)?Troisième Personne.
To say "third" in "third party" in a legal sense is a fraction is a stretch as typically there are over 9.7 billion persons who could be considered a "third party"!
If you go through the contortions to claim it's "a fraction", then why is it not also "tierce personne" for grammar, there are also three groups referring to the same Me/You/Other(s) portions of the population?
Why is it not "tierce" when it's referring to the third portion of a document?
It seems a lot more coherent to simply say that "you just have to know" when to use the adjective tiers/tierce. (You've so done elsewhere, as I linked in my previous comment.)