You know how sometimes you're staring at a problem or a math proof, and you know that you could find the solution? But maybe you don't go all the way and actually explicate the solution. There's just the feeling of existence that's present.
I often feel a similar black-box-y feel when I'm thinking about justifying certain rationality techniques. As in, there's effort I could put in to "translate" whatever technique I'm using into a more Standard Ontology which makes it sound Reasonable. But sometimes, I just deal with existence proofs.