Qi

Strength resides where one's ki is concentrated and stable; confusion and maliciousness arise when ki stagnates.~Morihei Ueshiba

Qi, [also qì or ch'i; gi in Korean, and ki in Japanese] in traditional Chinese culture is an active principle found in any living thing. Qi literally translates as "breath", "air", or "gas", and figuratively as "material energy", "life force", or "energy flow". It is the central underlying principle in traditional Chinese medicine and martial arts. It is comparable to concepts of prana in Hinduism, pneuma in ancient Greece, mana in Hawaiian culture, lüng in Tibetan Buddhism, ruah in Hebrew culture, and vital energy in Western philosophy.

Quotes

The very name Aikido indicates its dependence on the laws of nature, which we term ki. Aikido means the way to harmony with ki. That is to say, Aikido is a discipline to make the heart of nature our own heart, to understand love for all things, and to become one with nature. Techniques and physical strength have limits; the great way of the universe stretches to infinity.
- Koichi Tohei, in Book of Ki : Coordinating Mind and Body in Daily Life (1976), p. 106

Extend ki.
- Koichi Tohei, in Ki Sayings (2003), § 1 : Motto

The absolute universe is one. We call this ki. Our lives and our bodies are born of the ki of the universe. Study thoroughly the principles of the universe and practice them. We are one with the universe. There is no need to despond, no need to fear. The way we follow is the way of the universe, which no difficulty nor hard-ship can hinder. Let us have the courage and say, "if I have a clear conscience and a calm spirit, I dare to face an enemy of ten million."
- Koichi Tohei, in Ki Sayings (2003), § 3 : The way to union with ki

Ki tests are not founded on the idea of testing for strength or weakness. The most important factor in ki testing is to accurately inform the person of the state of his or her mind. Thus, the person performing ki tests must truly understand and exhibit oneness of mind and body from the outset and then perform the tests correctly.
- Koichi Tohei, in Ki Sayings (2003), § 22 : The treasure of ki testing

"Do nothing" does not mean "don’t do anything", "mu" or "nothing" is not merely the lack of something. It refers to the state in which the mind has grown imperceptibly calm, and then allowed things to take their natural course. If you can leave things entirely up to the universe and not put up the slightest resistance, then you can pass the highest level of ki tests.
- Koichi Tohei, in Ki Sayings (2003), § 24

The ki of the universe is absolute. There is no such thing as strong ki verses weak ki. Correctly speaking, ki is strong only because it is strongly extended, weak only when it is not extended strongly. There is no limit to how strong your ki can become if you train yourself to extend ki strongly.
- Koichi Tohei, in Ki Sayings (2003), § 31

The ki of the universe has never for a moment stopped moving. We call this continuous growth and development. Do you think it strange that human beings seem to be the only one trying to stop the movement of ki?
- Koichi Tohei, in Ki Sayings (2003), § 34

The purpose of ki-aikido is not self-defence; that is a mere by product. It is far more important to learn to control the mind and body. It is too late to try to calm the mind after you take up the sword. First you must calm the mind and then take up the sword. When you raise the sword up overhead, do not cut your ki. Continue to calm the mind by half, half, half and create a living calmness in that infinite reduction. When practicing cutting with the sword, you will find infinitely more value in cutting just five to ten times with ki fully extended, than you would in cutting a thousand time with mere physical strength.
- Koichi Tohei, in Ki Sayings (2003), § 50

Some people are quick to find reasons and excuses why they cannot do things. This cuts their Ki and in times stifles their motivation altogether. Motivation is extending Ki, not receiving it. People today are more concerned with what they can get, than what they can give or do for others. That is why they cannot extend Ki.
- Koichi Tohei, in Ki Sayings (2003), § 51

In our techniques we enter completely into, blend totally with, and control firmly an attack. Strength resides where one's ki is concentrated and stable; confusion and maliciousness arise when ki stagnates.
- Morihei Ueshiba, in The Art of Peace (1992)