Supreme Court Justice Potter Stewart was praised and excoriated after he observed in a 1964 pornography case that although he could not come up with a concrete definition of pornography, still, "I know it when I see it."
He was praised, I imagine, for his candor -- everyone has their own threshold and standards for what is clearly a personal persuasion. But he was excoriated, I imagine, for introducing into a discipline that prides itself on its concrete precision (the law) a factor that is clearly not at all precise ... where would the law and its punishments be if everyone were free to apply his or her own yardsticks? It's a frightening and anarchic prospect, one open to whimsical and far-reaching depredations.
I know it when I see it. "Pornography," yes, but also "love" and "freedom" and "justice" and "injustice" and "joy" and "enlightenment" and "compassion" and ... well, pick your poison. Every attempt to squeeze such things into some legally-defined and universally agreed-to box throws up wriggling tendrils that don't quite comply with the edges of that box. At which point, the human mind falls back on its exhausted and understandable haunches: "I know it when I see it."
And "seeing," after all, "is believing."
But for the less careless and the more determined, I think, "seeing is believing" overstates the case considerably. Seeing convinces me, but that's no reason to therefore "believe." Seeing is clear and convincing and lacks doubt whereas "believing" implies doubt by its very nature. Believing is an attempt to put things in a legalistic box ... from which, because human beings are more interesting than the boxes that they create, tendrils of exception wave their wriggly arms. And knowing (as in, "I know it when I see it") is often, if not always, confused or conflated with believing.
The important part for the less careless and more determined is the willingness to examine. Examining someone else's beliefs is fun for a while. But examining our own beliefs and our own knowing is hard work ... hard, but in the end, the only sort of examination likely to bear more than a tentative fruit.
I know it when I see it.
Seeing is believing.
Seeing is seeing -- useful in its time but ...
Knowing is knowing -- useful in its time, but ...
And if you believe that, I'll tell you another one.