The main purpose of this study was to verify the process of formation and development of the concept of aiki used by Morihei Ueshiba, the founder of Aikido, and his disciples. The main points can be summarized as follows:
1. The term aiki has been used to refer to particular martial arts techniques and to a spiritual state that can be experienced by practicing Aikido. Morihei taught aiki as a technique, as shown in the memorandum of the Japanese Imperial Navy Admiral Isamu Takeshita around 1930. Kisshomaru Ueshiba, Morihei Ueshiba's son, also introduced these techniques in his book, “Techniques of Aikido” (1962), etc. However, neither Morehei Ueshiba nor his son explained about aiki in detail. Kenji Tomiki and Gozo Shioda used aiki as a term of technique, but they do not seem to have taught techniques under the name of aiki.
2. Onisaburo Deguchi, the head of Omoto-kyo, used the expressions “the union between a kami and a mortal” in 1921 and “the great love of the kami” in 1935, which Morihei later emphasized in relation to aiki. Omoto-kyo heavily influenced the building of Morihei's thoughts on aiki and Aikido. Aiki was likened to the great love of the Universe, Heaven and Earth, or the kami who nurtures all nature and mortals. In short, a) aiki is the union between the kami as love, and mortals, hence the practice of aiki is the purification of mind and body; b) the practice of Aikido creates a paradise for mortals on earth; c) because the kami does not oppose anyone, a practitioner does not oppose in Aikido. Morihei's thought influenced the policy of the succeeding organization of Aikido through Kisshomaru.
3. Morihei's four main pupils inherited his thoughts through several arrangements. Shioda explained aiki as “a technique for following the laws of nature”. Tohei insisted that aiki is “the union between the ki in heaven and earth and a mortal”. Sunadomari interpreted aiki as a combative technique and a divine work. Tomiki understood the term in two ways: one is a technique that falls into the category of kuzushi (balance-breaking), and the other is the unity of ki (energy) between nature and man. As to the way that Aikido should develop in the future, we need to study further Morihei's thoughts and their development under his pupils.