Sometime in the early-to-mid-1990s, I checked out a hardcover SF anthology from a public library. I don't remember the title, the cover art, the unifying theme (if there was one), or much else. I do know it was a collection of stories by various authors, and that at least some of the stories had an "old-fashioned science fiction" feel to them, making me think they had first been published decades earlier -- perhaps in the 1950s? (But I can't remember any exact dates provided at the front of the book.)
The story that I remember best is the one I'm trying to track down now. Since I've never run across it again in the last twenty years or more, I strongly suspect it was not written by any of the especially famous SF authors who were already active in the mid-20th Century (such as Isaac Asimov or Poul Anderson).
As the story begins, the protagonist -- an adult male who basically seems to have spent his entire life until now as a very ordinary fellow, rather than a brilliant scientist, or a highly trained military officer, or anything similarly impressive -- has already won an important contest organized by some alien visitors to Earth who, after making a peaceful first contact, insisted that they wished to test our species for a person who would qualify as "an Ethical Absolute." (Or some similar phrase -- I don't swear those were the exact words.)
The contest, or at least the first round of it (there may have been later rounds?), was handled in the form of a written exam which presented questions calling for carefully thought-out answers. Those questions dealt with tricky matters of ethics. I don't remember any of them exactly (and I think we were only given a couple of examples), but they probably included such touchy matters as whether or not imposing the death penalty was justifiable in a modern, civilized society.
Our hero was considered to have aced the contest -- the only human being to have done so. He had written out a catchall response for all these questions. Something along the lines of: "Sufficient data has not been provided for me to be able to properly answer these questions as they now stand." (In other words, he was suggesting the following: "Whether an extreme action was justified would all depend upon the nitpicking details of a particular situation! So give me some context!")
As I recall, I was far from clear on how the aliens knew this was his "sincere and honest opinion" on each of the points raised in the test, instead of its just being an attempt to make a very good impression as a calculated psychological gambit which masked his true opinions. But they did seem sure of that. (Apparently they were correct.) Be that as it may, the protagonist was subsequently selected for a special honor which involved making a big trip (on an alien starship) to some other civilized world. I forget just what he was expected to do when he got there.
Some or all of the adult members of other alien races/cultures (I think there was more than one such race/culture tied together into one larger civilization) already had significant psychic abilities. I am almost sure that this included telepathy, and possibly telekinesis. But the aliens were under the impression that the human race had not yet developed such abilities.
This impression, the reader soon learned, was inaccurate. Somehow or other, at least one government agency on Earth (I don't remember if there was a world government yet) had already developed ways to awaken similar psychic abilities in at least some humans, but this R&D project was being kept a tight secret, and the researchers had also found a way to implant something in a human subject which would allow him to switch these powers on and off at will.
Our hero, the contest winner, had gotten the relevant treatment during the prep time before he left Earth. I believe that in the mode he generally used, he did not seem to have psychic abilities (from the point of view of psychic alien observers), but his thoughts were screened or "jammed" or something so that the nonhumans traveling on the same ship with him would not be able to easily read his mind and learn the truth.
All of the above was mostly the set-up . . . stuff which was established, in the early pages of the story, as having "already happened before we tuned in." Most of the story takes place aboard the interstellar vessel which is transporting our hero (and many other sentient beings) to some other star system. Various cloak-and-dagger shenanigans take place during the trip (the details largely escape me), during which our hero finds it necessary to use his secret psychic powers to save his own life, and probably the lives of some of his new (alien) friends, in some action scenes.
At the end of the story, our hero tells some of the "good" aliens (now that the bad ones have been identified and subdued) about the existence of his secret psychic powers. He assures his audience that human leaders now know that they could give all of their citizens such powers, but that they are understandably afraid of how such things could be abused by many selfish or immature people, and so they are exercising a ton of careful self-restraint for the time being, to avoid self-inflicted catastrophe. One of his alien listeners says admiringly (this is not an exact quote): "I said so! Ethical Absolute!" (Or whatever the exact catchphrase was which was attached to winning the aforementioned contest.)