I know you are, but what am I?

I know you are but what am I is the ultimate comeback for kids below the third grade. When a group of kids (mostly males) get together with minimum supervision, they start to pick on each other (in an Alpha Dog way). Scathing soul-shattering insults will be hurled back and forth like this:

First kid: "You're a doo-doo head."

Second kid: "You wet the bed."

First kid: "You kissed your sister."

Second kid: "You're a gorilla's butt." (major laughter from surrounding crowd... major score!)

First kid: "I know you are, but what am I?"

Now, no matter what the second kid says, the speaker of the ultimate comeback will just deflect any insult right back to the source. After four or five futile attempts, the second kid gives up in a huff, mentions "That's so lame!", then wanders off to throw dirt at the icky girls.

There actually is a comeback to the almighty "I know you are, but what am I?"

"It takes one to know one."

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This was demonstrated in the 'lemon tree episode' of the Simpsons.

Bart and co. gather around the Springfield lemon tree to bask in the citrus-y goodness. A few metres away, just over the Springfield/Shelbyville border, some hoodlums gather and the insults fly:

Shelbyville Kid: You're a garbage man.
Bart: I know you are, but what am I?
Shelbyville Kid: A garbage man.
Bart: I know you are, but what am I?
Shelbyville Kid: A garbage man.
Bart: I know you are, but what am I?
Shelbyville Kid: A garbage man.
Bart: Takes one to know one!

*stunned silence*

Nerd: Checkmate!

"I know you are, but what am I?" is perhaps the ultimate existential question.

For I know that I am -- cogito ergo sum, I perceive my existence, and in so perceiving am aware that I exist -- but I cannot be certain that what presents itself to me as the nature of my existence is reality at all. Perhaps I am simply an elaborate computer simulation designed to believe that it is alive, with the electrical charges understood to be electrosensory perceptions of sight, sound, touch, taste, simply being something like bits and bytes being whirled around some great CPU. Perhaps I am in the Matrix. Perhaps I am a brain in a jar.

But the one thing of which I can be certain beyond the fact of my existence is that I am not all that exists. For I perceive otherness itself, the existence of other minds with which I may interact, but which are not my own, or which lie beyond my control. If reality is as it presents itself, there are other beings. If I am a simulation, there is a great programmer who created it. Perhaps all of other minds I believe myself to be encountering are simply different facets of a single other mind, in which event, to the extent that any other being is looking at these words crafted in mind, these others are all you. And so I know, at the least, that you are!! Perhaps one of you, perhaps countless multitudes. Perhaps you and I are simply fragments of some even greater mind, but even so within that context I experience myself and not you, you experience yourself and not me, and so at some level, distinct from me, as much as I know that I am, I know you are.

But, the question ever lingers, what am I?