The Moths and the Flame by Farid ud-Din Attar

(Courtesy of the Grand Mufti. Or to be precise, the Grand Mufti's new father-in-law. As in he didn't have one before but now has one, rather than a different one).

Moths gathered in a fluttering throng one night
To learn the truth about the candle light,
And they decided one of them should go
To gather news of the elusive glow.
One flew till in the distance he discerned
A palace window where a candle burned --
And went no nearer: back again he flew
To tell the others what he thought he knew.
The mentor of the moths dismissed his claim,
Remarking: "He knows nothing of the flame."
A moth more eager than the one before
Set out and passed beyond the palace door.
He hovered in the aura of the fire,
A trembling blur of timorous desire,
Then headed back to say how far he'd been,
And how much he had undergone and seen.
The mentor said: "You do not bear the signs
Of one who's fathomed how the candle shines."
Another moth flew out -- his dizzy flight
Turned to an ardent wooing of the light;
He dipped and soared, and in his frenzied trance
Both self and fire were mingled by his dance --
The flame engulfed his wing-tips, body, head,
His being glowed a fierce translucent red;
And when the mentor saw that sudden blaze,
The moth's form lost within the glowing rays,
He said: "He knows, he knows the truth we seek,
That hidden truth of which we cannot speak."
To go beyond all knowledge is to find
That comprehension which eludes the mind,
And you can never gain the longed-for goal
Until you first outsoar both flesh and soul;
But should one part remain, a single hair
Will drag you back and plunge you in despair --
No creature's self can be admitted here,
Where all identity must disappear.