The true mind can weather all the lies and illusions without being lost. The true heart can touch the poison of hatred without being harmed. Since beginningless time, darkness thrives in the void, but always yeilds to purifying light.

That's what the lion turtle said. (When he first met Aang. The second part with Aang trying to "purify" Ozai is one I didn't memorize.)

The lion turtle was telling him not to rely on his past lives (power of the avatar) but on the power within himself. Ever since day one, Aang has had unbreakable spirit. the serpents pass was an example of his spirit showing more tragically, but he overcame it and became strong.

With an unbendable spirit, he was able to purify light, where darkness goes to show the void. (that's the rest of what the Lion turtle said about purifying light. )

In a sense, the Lion turtle combined the four philosophies into one, with ying and yang.

It was quite poetic, and once I understood, I was pleased with avatar even more.

And I agree with AS on what she said as well.

I believe the hard to understand part at the end was:

"In the era before the Avatar, we bent not the elements, but the energy within our (selves/senses, still can't say which).

To bend another's energy, your own energy must be unbendable, or you will be corrupted and destroyed, etc."