The concept of ki is one of the most difficult associated with the philosophy and practice of aikido. Since the word “aikido” means something like “the way of harmony with ki,” it is hardly surprising that many aikidoka are interested in understanding just what ki is supposed to be. Etymologically, the word “ki” derives from the Chinese “chi.” In Chinese philosophy, chi was a concept invoked to differentiate living from non-living things. But as Chinese philosophy developed, the concept of chi took on a wider range of meanings and interpretations. On some views, chi was held to be the most basic explanatory material principle – the metaphysical “stuff” out of which all things were made. The differences between things depended not on some things having chi and others not, but rather on a principle (li, Japanese = ri) which determined how the chi was organized and functioned (the view here bears some similarity to the ancient Greek matter-form metaphysic).

Modern aikidoka are less concerned with the historiography of the concept of ki than with the question of whether or not the term “ki” denotes anything real, and, if so, just what it does denote. There have been some attempts to demonstrate the objective existence of ki as a kind of “energy” or “stuff” that flows within the body (especially along certain channels, called “meridians”). So far, however, there are no reputable studies which conclusively demonstrate the existence of ki. Traditional Chinese medicine appeals to ki/chi as a theoretical entity, and some therapies based on this framework have been shown to produce more positive benefit than placebo, but it is entirely possible that the success of such therapies is better explained in ways other than supposing the truth of ki/chi theory. Many people claim that certain forms of exercise or concentration enable them to feel ki flowing through their bodies. Since such reports are subjective, they cannot constitute objective evidence for ki as a “stuff.” Nor do anecdotal accounts of therapeutic effects of various ki practices constitute evidence for the objective existence of ki – anecdotal evidence does not have the same evidential status as evidence resulting from reputable double-blind experiments involving strict controls. Again, it may be that ki does exist as an objective phenomenon, but reliable evidence to support such a view is so far lacking.

There are a number of aikidoka who claim to be able to demonstrate the (objective) existence of ki by performing various sorts of feats. One such feat, which is very popular, is the so-called “unbendable arm.” In this exercise, one person,, extends her arm, while another person, , tries to bend the arm. First, makes a fist and tightens the muscles in her arm. is usually able to bend the arm. Next, relaxes her arm (but leaves it extended) and “extends ki” (since “extending ki” is not something most newcomers to aikido know precisely how to do, is often simply advised to think of her arm as a fire-hose gushing water, or some such similar metaphor). This time, finds it (far) more difficult to bend the arm. The conclusion is supposed to be that it is the force/activity of ki that accounts for the difference. However, there are alternative explanations expressible within the vocabulary or scope of physics (or, perhaps, psychology) that are fully capable of accounting for the phenomenon here (subtle changes in body positioning, for example). In addition, the fact that it is difficult to filter out the biases and expectations of the participants in such demonstrations makes it all the more questionable whether they provide reliable evidence for the objective existence of ki.

Not all aikidoka believe that ki is a kind of “stuff” or “energy.” For some aikidoka, ki is an expedient concept – a blanket-concept which covers intentions, momentum, will, and attention. If one eschews the view that ki is a stuff that can literally be extended, to extend ki is to adopt a physically and psychologically positive bearing. This maximizes the efficiency and adaptability of one’s movement, resulting in stronger technique and a feeling of affirmation both of oneself and one’s partner.

Irrespective of whether one chooses to take a realist or an anti-realist stance with respect to the objective existence of ki, there can be little doubt that there is more to aikido than the mere physical manipulation of another person’s body. Aikido requires a sensitivity to such diverse variables as timing, momentum, balance, the speed and power of an attack, and especially to the psychological state of one’s partner (or of an attacker).

In addition, to the extent that aikido is not a system for gaining physical control over others, but rather a vehicle for self-improvement (or even enlightenment (see satori)), there can be little doubt that cultivation of a positive physical and psychological bearing is an important part of aikido. Again, one may or may not wish to describe the cultivation of this positive bearing in terms of ki.