It has been said that the essence of combat lies in between sei and ki (or the regular and the irregular) and without attaining the ability of changing sei into ki and ki into sei how can one attain victory?

In a display of kata a judoka who exhibits the right magnitude of stillness and movement employs the concepts of sei, or passivity, and do, or activity, in their psychological and physical attitude and feeling. The exponents of aikido, do not generally realise that, in spite of the great emphasis placed on ki, the Confucian reverse, ri, or reason, must be equally working if agreement of spirit and action is to succeed.

Whether they are stated or ambiguous, understood or disbelieved, celebrated or mysterious these things can be identified as functioning in all types of budo. Only through the course of these principles can any exponent demonstrate a quality of performance that gives the correct spirit to the material action or the necessary emotional mood to accompany its presentation.

A performance without any of these aspects appears as a 'dead' performance, the result being apparent in the lack of feeling' on the part of one or all performers. This defect indicates the lack of an 'inner life' in those who are exhibiting their skill, the 'inner life' that is an essential quality of all true experts.

“The great Tao flows everywhere. Through the Yin and to the Yang.
All depend on it, and none are refused.
It flows into its task, nourishing infinite worlds, clothing and feeding, yet it does not claim ownership or allegiance
Eternally without need, claiming nothing, it is ultimately humble.
Yet as all things naturally flow back into it, it is truly great.
The master therefore never tries to be great, and is thus truly great

Tao Teh Ching. 34: Inherent Perfection.”

So, the epitome of budo appears to be firmly stuck between ki and sei or between in (yin) and yo (yang). I think that those who study budo should think about these concepts both in connection to their daily training and in their daily life.

To begin to comprehend this, we first have to deepen our understanding of what is meant by in and yo. This is my understanding - as far as I can put it in words.

Yo, the active principle is the first thing to manifest in the cosmos it is the pouring forth of energy that can be characterised by a straight line. It is force, in particular a thrusting, driving or primal force.

In, the passive principle is the second thing to manifest in the cosmos (as a natural result of yo) it is the receiver of energy that accepts the force of Yo, and gives it a form, as a result harnesses the force and brings it to a useful completion.

One can clearly see the masculine and feminine principles within these concepts, and it is worth considering that physics has so far found that matter consists entirely of counterbalanced forces.

To take this further, anything which acts as a stimulus is Yo to the thing that it stimulates. If I go and drop a rock on someones head, then I am yo, they are In. But both In and yo contain the seeds of each other. Take the teacher-student relationship. The teacher is clearly Yo to the student, so the student is largely In to the teacher. But teachers learn from their students also, so we have the seed of yo within something that is In.

We could all examine the interchange of In & Yo to help find the seed of In within the Yo of Uke's attack. Or we could view this as finding the seed of Yo in the In of tori's avoidance/acceptance of the attack.

In a single technique uke is Yo as he attacks, tori is In as he avoids and changes to Yo as he strikes. uke becomes In as he falls over - (this is not a very successful way to be In)
in order to counter the technique uke must become In in a more controlled and successful way and transform again to Yo, thereby forcing tori to be In, so that he must fall over or counter the counter.

There are many tangents to follow, since anything can be classified by these concepts, and they are always relative and always changing. We are all In and Yo to each other in different ways at different times. It is very important to recognise the time to be In and the time to be Yo. The trouble is that In & Yo are such big concepts. They are the archetypal two sides of the coin that we see in everything. They become complex when you consider them in particular things, but are simple in and of themselves.

The Shushigaku school introduced a theory of reality or metaphysics foreign to the well-established Buddhist theories in Japanese thought. In particular, it analysed reality in terms of the dynamic between ‘configuration’ or ‘principle’ (ri) and ‘material energy’ or ‘vital force’ (ki). According to Shushigaku, ri gives the universe its structure and, since ri is also in the mind, it is the foundation of knowledge. By ‘investigating the nature of things’ we come to know ri, both in ourselves and in the things we study. Ki, on the other hand, was considered the basic stuff that is ordered by ri.

Although the notion of ri was known to the Japanese through Tendai and Kegon Buddhism, the neo-Confucians gave the term a distinctive emphasis. They embedded it into their understanding of the natural world. During the Tokugawa period there was a practical interest in better understanding nature; in the sixteenth-century traders, and missionaries from Europe had introduced some Western science. With the closure of Japan this contact was severely limited, although the occasional Dutch treatise on practical science or medicine did find its way into Japan.

For the most part, Japanese philosophers found the Shushigaku emphasis on ri to be overly abstract. To many, it seemed that ri was an unnecessary transcendent realm behind physical reality that could be known only through some mysterious half-contemplative, half-empirical study. In response, many Japanese thinkers treated it more like a strange phenomenon. Kaibara Ekken (1630–1714), for example, argued for the primacy of ki. To him, ki was the basic ingredient of reality and should be studied directly; ri was no more than the name for the patterns one could abstract from the behaviour of ki. Certainly from the perspectives of both medicine and the martial arts, ki became the more important category in Japan.

Ki is a very handy word because it has both a deep meaning connected with nature, and a light meaning which is used in daily life. It is difficult to pin a definition onto ki as a concept.

In Oriental thought, it is said that in the beginning there was chaos. The dust of chaos settled gradually to form the Sun, the Earth, the Moon and the Stars. On the Earth the elements united to become minerals, animals and vegetables. The chaotic state before the universe took shape is known as ki. So therefore it can be said that all things come from ki. This is the deep understanding of ki.

Ki itself does not begin or end. Its shape can change, but ki itself never does. We can see many things around us which are all made from ki. When they loose their shape then their elements can be said to return to ki.

In a book called Mysteries of the Martial Arts, Dr Clive Layton claims that Western scientists studying the phenomenon of ki have acknowledged an analogy between ki and the Spirit of God. This may suggest a similarity between the Eastern idea of ki and the Western idea of the Holy Spirit, or at least with something divine. If this is the case then ki most certainly can be understood in a metaphysical way. The fact of the matter is, that in spite of the differences of the Martial Arts, Religions, Philosophies or what ever, that one attempts to master, we are all searching for the same thing, a way to express the same fundamental truth that underlies all life.

A so called 'light' definition of ki could be a good feeling, a bad feeling, a great feeling, feeling nervous, being full of energy, or courage. These are all terms used in daily life and are all basic parts of ki.

As established previously, humans are said to be created from ki of the universe. This is both good and useful. While man receives ki, he is alive. Once deprived of ki, he dies, or rather looses his human form. For a religious man it may be seen as the soul struggling to unite with the spirit of God, as a Martial Artist it is the act of looking inwardly deeper. But as with all intelligent people, who feel, sense, realise and conclude through their daily activities that there is something else to life than just living it.