ἀγαθός (agathos) is the ordinary Greek word for good in the plain moral sense — good as in upright, virtuous, morally sound. It’s the word that does the everyday work of describing a good person, a good deed, a good character. And the most useful thing about it, for hearing the New Testament well, is what it reveals by its absence in a place you’d expect to find it.

Take “I am the good shepherd” in John 10. An English reader naturally assumes the “good” there is moral goodness — the shepherd who is kind, faithful, virtuous. If that were the sense, ἀγαθός is exactly the word the Greek would reach for. But it isn’t the word Jesus uses. He uses a different one, kalos, which means good in the sense of genuine, true, the real thing, the model of its kind — the shepherd as a shepherd is meant to be, set against the counterfeits. The contrast between the two words is the whole point. Had the saying used ἀγαθός, it would lean toward “the morally upright shepherd.” Using kalos instead, it leans toward “the true shepherd” — the genuine article over against the frauds and hirelings Jesus immediately starts describing, the hired men who run when the wolf comes.

So ἀγαθός functions here as the road not taken, and that’s its value. By noticing which word the text didn’t choose, you recover what the word it did choose is actually claiming. The good shepherd isn’t first being praised for his virtue; he’s being identified as the real one against the false ones. That distinction unlocks the Old Testament background — Ezekiel’s true shepherd against the failed shepherds of Israel — that the saying is reaching back to. If you flatten kalos into ἀγαθός, into mere moral goodness, you lose the contrast that the entire passage is built around, and you miss the prophet standing behind it.

This is one of those places where Greek draws a line English blurs. English has a single broad word, “good,” that has to cover both the moral sense and the “genuine, fine, true” sense. Greek keeps them apart: ἀγαθός for the one, kalos for the other. Knowing that ἀγαθός exists, and that the text declined to use it, is how a reader without Greek can still feel the precision of the choice. “I am the good shepherd” means “I am the true shepherd, the genuine one” — and the simplest way to be sure of that is to notice that the ordinary word for morally good was available, and Jesus reached past it.