Taijiquan, or "Supreme Ultimate Fist", is the Chinese internal art that utilizes many philosophical principles in its practice - in fact, it's these very philosophies that ensure the successful application of the techniques. Based mainly on Taoist philosophical principles, Taijiquan is in effect a living, breathing showcase for their everyday usage. First and foremost would have to be the theory of Yin and Yang, the opposing yet complimentary forces that are said to be in everything in the Universe. Many would say that Yin and Yang are opposites; while that is true, it's also only a half-truth. The secret is in knowing that they are also dependent upon each other for their very existence. Without Good you cannot have Evil; without light there is no darkness. It's important to learn how to conceptualize this theory in Taijiquan because every movement that you'll ever learn employs this principle. A corollary to this principle is that the Yin-Yang is in constant motion: the Yin is becoming Yang as the Yang is becoming Yin. There is no static state in which they are 100% themselves. Rather, they are always changing, always recognizing a little Yang in the Yin and vice-versa. It is actually at the point of intersection - when they are equally balanced - that you find your greatest strength and stability. Conversely, you seek to catch your opponent at the moment of their greatest swing to either full Yang or full Yin. Wu-Wei, or "the action of non-action", may sound like an outright contradiction, but like so many things in Taijiquan and Taoist philosophy it has a deeper, hidden meaning. Taijiquan prides itself on not exerting effort; it is said that you should be able to move 4,000 pounds with 4 ounces of effort. The trick to this is that ol' Yin-Yang principle again - if you catch your opponent when they're "floating" you need only apply a light tap to a strategic spot and their mass and momentum will do the rest. It's a case of doing very little but waiting and watching, but the end-results are more than satisfactory. You've "acted" without really "acting". You've merely helped your opponent go where they wanted to go in the first place. It's all about going with the flow, aligning yourself with the Tao. Grounding is as much a mental and emotional trait as it is a physical one. We seek to anchor and purify our mind, eliminate emotions in a conflict and physically drop a "root" to achieve grounding. This stabilizes us, enabling us to take advantage of both mental and physical imbalances in our opponents. Watch a toddler when he wants to be picked up: he's as light as a feather. Then watch as he throws a tantrum and doesn't want to be touched - it will seem as if he weighs a ton. Same child, same adult, but the intent has changed - he's grounding. Taijiquan masters can implement this ability at will; they can become so heavy that several people cannot push them over, yet they can move as quickly as a feather in a tornado when they so choose. Relaxation is one of the most-advertised health benefits of learning Taiji, but it is equally important in the martial applications. You remain soft, fluid and relaxed, even while striking, until the very instant that you reach your target - then you become hard. "Iron fist in a velvet glove" is a saying in Kung-Fu that is totally appropriate to Taijiquan's use of this principle. Also, it is through being relaxed that you can move so quickly without strain or effort. You develop a whip-like striking ability, which when combined with the internal energy - qi - enables you to produce jing, a sort of combination of the physical and energetic.