The alethiometer—the titular golden compass—represents children’s unique ability to discern truth. The Alethiometer is a gold instrument with pictures and three needles that, if one knows how to read it, will answer any question truthfully. The fact that Lyra can read the alethiometer without study (Farder Coram notes that it takes adults years and a specific reference guide to learn to read it) suggests that given the way the novel conceptualizes childhood and innocence, the ability to ascertain the truth like this is something unique to children. In this way, the alethiometer more broadly makes the point that because of their innocent nature, children can, when given the correct tools, be the least biased and most effective judges of morality or truth.
The Alethiometer Quotes in The Golden Compass
"Yes. Lyra has a part to play in all this, and a major one. The irony is that she must do it all without realizing what she's doing. She can be helped, though, and if my plan with the Tokay had succeeded, she would have been safe for a little longer."
"Because, Iorek, listen: I got this symbol reader that tells me things, you see, and it's told me that there's something important I got to do over in that village, and Lord Faa won't let me go there. He just wants to get on quick, and I know that's important too. But unless I go and find out what it is, we might not know what the Gobblers are really doing."
"How do you do that?"
"By not being human," he said. "That's why you could never trick a bear. We see tricks and deceit as plain as arms and legs. We can see in a way humans have forgotten. But you know about this; you can understand the symbol reader."
"That en't the same, is it?" [...]
"It is the same," he said. "Adults can't read it, as I understand. As I am to human fighters, so you are to adults with the symbol reader."
"Yes, I suppose," she said, puzzled and unwilling. "Does that mean I'll forget how to do it when I grow up?"