gnu - He's saying that those who categorically refuse to look for something (because they don't "believe" in it or aren't interested) are extremely unlikely to find it. This would be true of anything one could look for, material or otherwise. If you have no interest in it, and you are unwilling to do any work to find it, then you very probably will not find it. ;-)

People who are unwilling to look within themselves, for instance, are most unlikely to find what's in them, aren't they? I've known thousands of people like that. Their attention is entirely on what's outside of them. Their criticism is entirely for what's outside of them. Their desires are entirely for what's outside of them. They (some of them) even tend to imagine a God that's entirely outside of them! They will never look within themselves...the reason, I think, being: they're afraid of what they might find. Or else they're just too distracted by the outer world to ever even think of looking within.

That's what the philosopher was referring to, I think, when he said, "The unexamined life is not worth living."