Fundamental Theories of Hap Ki Do

Harmony Theory (Hwa):
The harmonizing of body, mind, and spirit within one's environment is a fundamental long-term goal of Hapkido training.  With harmony, technique becomes fluid, continuous and instinctive.  Every action becomes purposeful and perfectly linked to the moment.  The martial artist no longer responds in the pattern of "think, then react" - thought and action merge into a single, purposeful act.  This state of harmony is not just for fighting, but should pervade all aspects of one's life.  If you are truly in harmony with yourself and your environment, it is likely that you'll never need to fight.  Closely related to the Harmony Theory are the concepts of Empty Mind (or No Mind) and Total Awareness. 

Empty mind or no mind is a concept involving mental preparation, which is an outgrowth of Zen Buddhism.  It is characterized as centering, emptying, or quieting the mind, and is often an objective of meditation.  This centering of the mind heightens awareness and relaxes musculature, thereby decreasing response time.  You move, think and respond more quickly.  Your mind is relaxed, free from emotion and is better able to concentrate on the matters at hand.  It is described as "being in the moment" or "in the zone".

Water Theory (Yu):
The ancients observed that water was a powerful force.  Water always found the flaw in a vessel and could penetrate the smallest hole.  Water flowing in a river always exerted constant pressure, eventually overwhelming whatever stood in its path.  Over time, a single persistent drip could wear a hole through stone. 
Key qualities of Water Theory:

Constant Pressure	Refers to constantly striving toward you goals.  In terms of self-defense, this means putting constant physical and psychological pressure on your opponents, never allowing them to recover once you have the advantage.
Penetration	Means breaking down or passing through obstacles that prevent you from attaining your goals, seizing the smallest opportunity.
Persistance	Constantly directing you resources against an obstacle until it crumbles, never giving up, no matter how long it takes.
Adaptability	Adjusting positively to the changing circumstances of life, and finding the optimum path, while always maintaining you true nature.
Constant Flow	Maintaining a continuous flow of energy as you engage an obstacle.
Softness	Cultivating the loving, gentle way; and blending with the greater flows of energy you might encounter.

Circle Theory: (click to read more)
The circle is a universal symbol found in many cultures.  It has come to symbolize many concepts including:
totality; wholeness; original perfection; self-containment; the Self; infinity; eternity; recurrence, endless movement; perfect unity; fulfillment; and the cycles that define the universe. 
In Buddhism, the circle represents the totality of existence.  In Zen Buddhism, the empty circle represents enlightenment.  In Taoism, it symbolizes the origin of all things. 

The circle has tremendous relevance in HapKiDo's techniques.  Ancient martial artists believed that circular techniques and movements were effective because they harmonized with one's opponent, one's self and the universe.  If one was not at harmony when fighting, the chances of defeat were much greater, since one would not only be battling an opponent, but also one's self.  The technical aspect of the circle is of major importance within HapKiDo.  Circles are found in almost every technique.