Duality, a basic premise of the Taoist philosophy. While duality is only an aspect of the numerology of taoism (see the Tao of Health, Sex, and Longevity), it is one of the most symbolized aspects of Taoism in the form of yin-yang.
9/11 was a natural consequence of Natural Laws, some may call it Karma. by there definition in the Tao Te Ching, Good and Evil are indifferent when seen as deviation from the center. Stay in the center my friend(this was part of the message of all the great sages:Christ, Buddha, Lao Tzu, etc...)
The Tai Chi represents the nature of perceptual duality, among other things. I do not disagree with you S H, but When I see the combination of the opposites of the perceptual mind displayed in the Tai Chi I see the Reality of dualistic properties shown as ONE.