This contains spoilers for "The Western Air Temple", which aired in Canada. I'm sure you can find it somewhere….

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It is blocked by illusion.

Aang remembered being told that the greatest illusion humans entertained was that of difference. The four nations fought amongst themselves, divided by appearance, customs, mindsets and prejudice, and thus brother fought against brother.

The people of the world were really just one nation, under all the distractions. He was the Avatar, the four elements united in one body, and he should understand that better than anybody. Aang had told himself that he would live by that doctrine: told himself over and over again.

There was no such thing as a separation between people, because everyone was the same. Easy enough to understand, of course.

And yet Aang had never felt a distance more vast than the one between himself and the Fire Nation Prince standing five feet away. Really, he was still trying to absorb the prince's words…. Pleas, more like.

Zuko had just offered them his aid.

He was offering his services as a firebending teacher, to help them end this war.

He was offering to fight against his father…and for a moment, Aang glimpsed the hurt and confusion that Katara had spoken of. It had been healed over some, but the mark was still there.

He did not want to hurt Zuko any further. Some part of him wanted to reach out and welcome the lost traveler with open arms, assimilate him seamlessly into their mismatched family, act on his inquiry of friendship.

But there was another part of him, powerful and cold and vicious, that said Zuko could not be trusted: and after all the things the firebender had done to them, it was very easy to say which side of the argument was more logical.

Katara and Sokka were the same in their responses.

But Toph…Toph…

Toph had trusted him. She had vouched for him almost immediately, trying to clear the haze of past injuries away so that they could see the important role Zuko would play. But why? Aang had instantly wondered.

Why was it so easy for her to trust his words, the words of enemies in general, and not face the same doubt that the rest of them did? Aang turned to look at her out of the corner of his eye, small and sturdy with her burnt feet propped up. She blinked those creamy jade green eyes slowly, patiently.

The answer came to him.

She was blind.

She was blind to differences in eye color, hair color and skin color: blind to the preconceptions that came along with the word firebender, foreigner.

She did not judge people for their past acts, but only their current ones.

Toph did not see the differences that the rest of them did, because all human beings were alike, in her vision: they all had heartbeats. They all had hearts that could feel joy and sorrow and triumph and pain, that all beat with the same rhythm and the same life.

Toph saw only the unity and similarity between people.

She saw the truth. She saw that Zuko (their brother), in his honest words, was sincere in his desire to help them.

So, Aang concluded, Toph wasn't really blind.

("Sometimes I wonder who the blind one is…" she had muttered before as she stalked away.)

No, she was the one who saw clearest of all: Toph simply saw without prejudice, and illusion had no hold on her.

So when he turned to his teacher first for approval in his decision, his decision to let Zuko join, he knew just what she would say:

"Oh, go ahead and let him…" she grinned.

Aang smiled back, even though she couldn't tell.

Opening his eyes through her own sightless ones, she never ceased to amaze him.

And it is opened by understanding.