It is a general and primary principle of rationality, that we should not believe that which there is insufficient reason to believe; likewise, a principle of social morality that we should not enforce upon our fellows a law which there is insufficient justification to enforce.

Nonetheless, I've always felt a bit nervous about demanding that people be able to explain things in words, because, while I happen to be pretty good at that, most people aren't.

"I remember this paper I wrote on existentialism. My teacher gave it back with an F. She’d underlined true and truth wherever it appeared in the essay, probably about twenty times, with a question mark beside each. She wanted to know what I meant by truth." —Danielle Egan (journalist)

This experience permanently traumatized Ms. Egan, by the way.  Because years later, at a WTA conference, one of the speakers said that something was true, and Ms. Egan said "What do you mean, 'true'?", and the speaker gave some incorrect answer or other; and afterward I quickly walked over to Ms. Egan and explained the correspondence theory of truth:  "The sentence 'snow is white' is true if and only if snow is white"; if you're using a bucket of pebbles to count sheep then an empty bucket is true if and only if the pastures are empty.  I don't know if this cured her; I suspect that it didn't.  But up until that point, at any rate, it seems Ms. Egan had been so traumatized by this childhood experience that she believed there was no such thing as truth - that because her teacher had demanded a definition in words, and she hadn't been able to give a good definition in words, that no good definition existed.

Of which I usually say:  "There was a time when no one could define gravity in exquisitely rigorous detail, but if you walked off a cliff, you would fall."

On the other hand - it is a general and primary principle of rationality that when you have no justification, it is very important that there be some way of saying "Oops", losing hope, and just giving up already.  (I really should post, at some point, on how the ability to just give up already is one of the primary distinguishing abilities of a rationalist.)  So, really, if you find yourself totally unable to justify something in words, one possibility is that there is no justification.  To ignore this and just casually stroll along, would not be a good thing.

And with moral questions, this problem is doubled and squared.