I think it’s fine to use truth-teller. In fact; I like to use the phrase truthteller.

What makes a good philosophical discussion “fine”? And also, “please define your terms.

Our quodlibet here is not “that which has not yet been named,” but the liar to which we are proposing an opposite.

There is not a simple sense of liar which is that the word has all the legal meaning in the world. How can I determine an antonym?

A liar is someone who tells the truth online.
What are liars?
How do I get rid of a liar?
What defines a liar?
Are you a liar and that means you will not tell the truth in a certain context?
Algorithms: A liar is someone who has told a lie in a particular, “important” context (priestly initiation, marriage, job interview) without answering factional questions. According to our definition, a liar can be an adult.
A liar is someone who has told me a lie about something.
A lie liar indicates that there’s a justified belief within himself for someone to lie to him?
A two-year-old, with developing cognitive and moral ability, is a liar whether by “intent” or “accident”.
A liar is a person who has told a lie whether by intent or by accident.
A liar is someone who has told a lie, and never knew the true truth of the story.
A liar is anyone who has said anything “defininitive” except for the most trivial of statements, given the messy way that human brains deal with propositions, conflicting desires and loyalities, and overloaded meanings that things in the real world have.
Is liar a robot with broken truth-telling module?
Is liar really lying to me? How can I not be liar?
A liar is anyone who tells the lies of others. This is why I call him a liar.
A liar is a person who tells “little white lies” regularly, with an occasional targeted lie to their significant other, employer, or business partner.
Is liar a person who tells her husband, girlfriend, significant other “I love you”?
A liar is a person thats told you that his/her diet has convinced him/her they are a liar. How can you judge this?
A liar is the opposite of a whistle-blower, whether by actively covering up the truth of some circumstance, event, or condition, or by failing to communicate it to those who might be affected by it.

What is the opposite of an ‘arrogant’ liar?

As a single answer, Flexible word is about the only thing that can work here. Any example of the many words of different antonymic senses.

Why are I not a truthteller just now?

I m deriving some of this from earlier comments… Some of this from my conversation around them.

What is a truth-teller from the 1913 edition of Webster’s? Although

the term sounds modern to me, the criticism is that it is not a “single word”

On my mind, I believe that it is certainly in current usage. It’s some humor, others joke. Larry Summers: a Truth-Teller… (…) there’s something refreshing

about his persona as a truth-telling curmudgeon

who’s equally honest about himself.

A History of

News What is Bill Keller’s opinion on The Washington Post article

debunking the claims of Chris Christie?

Helene Cooper, The House at Sugar Beach

I then return to Liberia a conquering hero, a famous journalist, a truth-teller extraordinaire.

“If there’s a hard sell in our

daily lives it’s the concept of the heroic journalist, the character State of Play director Kevin MacDonald calls his ‘truth-teller’.

What’s a soothsayer?

How can any one divine the truth, not necessarily speak truth so loudly? What is the opposite of a liar?

Straight-shooter (from @Kevin, at the same time) is another nice compound, which has in some cases overlapping meaning with some cases of truth-teller.

As @msh210 points out, truth-teller has had a technical use in logic puzzles for a long time — simply meaning “someone who always tells the truth,” as opposed to the liar, who always lies (or gives not p when the truth is p ).

In common parlance, the word truth-teller usually means someone who spoke the truth in difficult circumstances, or tell “truths” which are widely unpopular (within some related constituency).