The sixth chapter is arguably the most important in the entire Art of War for understanding Wudang combat philosophy. It teaches the principle of attacking emptiness and avoiding fullness, the imperative of imposing your will on the enemy rather than reacting to his, and the ideal of becoming formless — like water, which shapes itself to the terrain it crosses. This chapter contains the text's most explicitly Daoist imagery.

The title 虚实 (Xūshí) pairs 虚 (Xū, empty/void/insubstantial) with 实 (Shí, full/solid/substantial). The interplay of emptiness and fullness — where to find them, how to create them, how to exploit them — is the chapter's subject.

Arrive First, Control the Ground

The chapter opens with a principle of initiative: whoever arrives first at the field of battle and awaits the enemy will be fresh; whoever arrives second and must hurry into combat will be exhausted. The skilled combatant imposes his will on the enemy and does not allow the enemy's will to be imposed on him.

In Tuīshǒu, this translates directly: the practitioner who controls the spatial relationship — who establishes contact on his own terms, at the distance and angle of his choosing — dictates the exchange. The reactive fighter is always one step behind.

Strike Emptiness, Avoid Fullness

The chapter's tactical core: attack where the enemy is empty (虚 Xū); avoid where he is full (实 Shí). Advance irresistibly by targeting weak points. Withdraw safely by moving faster than the opponent can pursue. The general skilled in attack leaves the opponent unsure what to defend; the general skilled in defense leaves the opponent unsure what to attack.

This is the foundational principle of all internal martial arts striking. In Wudang combat, the practitioner does not oppose force with force — that is striking fullness with fullness. Instead, the practitioner reads where the opponent's structure is weak, where his weight is committed, where his attention is focused — and strikes the gap. Diǎnxué (点穴, vital point striking) in Tàiyǐ Wǔxíngquán is the most refined expression of this principle: targeting the body's precise points of vulnerability.

The Water Metaphor

The chapter concludes with the text's most Daoist passage: military formations should be like water. Water naturally flows downhill, avoiding heights and filling low places. Likewise, the army avoids strength and strikes weakness. Water shapes its course according to the ground; the soldier shapes his victory according to the enemy. As water retains no constant shape, in warfare there are no constant conditions. The commander who can modify his response to match changing circumstances may be called a "heaven-born captain."

This is virtually indistinguishable from the Dàodéjīng's teaching on water. Chapter 78 of the Dàodéjīng teaches that nothing is softer than water, yet nothing overcomes the hard and strong more effectively. The entire Wudang approach to combat — yielding to redirect, flowing around resistance, finding the path of least resistance to the opponent's center — is the martial embodiment of this principle.

Formlessness

The chapter teaches that the ultimate tactical disposition is to have no fixed form (无形 Wúxíng). When your disposition is formless, the deepest spy cannot perceive it and the wisest strategist cannot counter it. Everyone can see the tactics that produce a specific victory, but no one can perceive the formless strategy from which the victory emerged.

In Wudang practice, the highest level of skill is described as "no technique" (无招 Wúzhāo) — the practitioner responds so naturally and spontaneously that no pattern can be identified or anticipated. This is the martial expression of Wújí (无极) — the limitless void from which all form emerges.