Gung fu, the centre of the Oriental arts of self-defence, is a philosophical art that serves to promote health, to cultivate the mind, and to provide a most efficient means of self-protection. Its philosophy is based on the integral parts of the philosophies of Taoism and Ch'an (Zen) - the ideal of being harmonious with and not against the force of the opponent. Just as a butcher preserves his knife by cutting along the bones, a gung fu man preserves himself by complementing the movements of the opponent.
The word gung fu means "discipline" and training toward the ultimate reality of the object - be it health promotion, mind cultivation or self-protection. There is no distinction to make between the opponent and the self because the opponent is but the other complementary (not opposite) part. There is no conquering, struggling, or dominating, and the idea is to "fit" harmoniously your movement into that of the opponent. When he expands, you contract; when he contracts, you expand. Expansion then is interdependent with contraction and vice versa, each being the cause and result of the other.
Gentleness/firmness is one inseparable force of one unceasing interplay of movement. If a person riding a bicycle wishes to go somewhere, he cannot pump on both the pedals at the same time or not pump on them at all. In order to move forward he has to pump on one pedal and release the other. So the movement of going forward requires this "oneness" of pumping and releasing. Therefore, gentleness alone cannot forever dissolve away great force, nor can sheer brute force subdue one's foe. In order to survive in any combat, the harmonious interfusion of gentleness and firmness as a whole is necessary, sometimes one dominating sometimes the other, in a wavelike succession. The movement will then truly flow, for the pure fluidity of movements is in their interchangeability.
So neither gentleness nor firmness holds any more than one half of a broken whole which, welded together, forms the true Way of martial art. The tendency to guard against is from getting too firm and stiff. Notice that the stiffest tree is most easily cracked, while the bamboo or willow survives by bending with the wind. This is why a gung fu man is soft yet not yielding, firm, yet not hard. The best example of gung fu is water. Water can penetrate the hardest granite because it is yielding. One cannot stab of strike at water and hurt it because that which offers no resistance cannot be overcome.
In actual application, gung fu is based on simplicity; it is a natural result of four thousand years of exhaustive experimentation and is of highly sophisticated complexity. All techniques are stripped down to their essential purpose without wastage or ornamentation, and everything becomes the straightest, most logical simplicity of common sense. The utmost is expressed and performed in the minimum of movements and energy.
The method for health promotion is again based on water, as flowing water never grows stale. The idea is not to overdevelop or to overexert but to normalise the function of the body.
Bruce Lee's handwritten essay on Gung Fu, untitled.