Since my aikido training took a hard turn almost 10 years ago, I have been examining the nature of ki and have come to these conclusions.
Ki is fundamental energy. It is the energy that forms atoms out of nothing, and it flows.
Fundamental energy has different properties depending how it manifests in the material world as we know it. These are to be found among the four forces in nature: gravity, electromagnetic, strong, and weak (the last two affect matter at the subatomic level).
I propose that there is a fifth way fundamental energy expresses that is so difficult to quantify that it has been put aside as either imaginary or magical or it is ignored altogether. It is called spiritual energy.
For sake of simplicity, I will refer to spiritual energy as ki.
Ki has properties just like the other four, but since electromagnetism and gravity are easily observable in our human scale I will compare ki mostly to them.
Some gross generalizations: Gravity affects mass and is connective. Magnetism is connective also but can be repellent as well. The larger and denser the mass, the greater the gravity; the more the flow of electrons the greater the magnetism. Magnetism easily overcomes gravity in some situations (refrigerator magnet sticks to the door, doesn't drop to the floor; levitation of objects) but is weaker in others (two inches away from the refrigerator and the magnet drops to the floor).
The way Gravity and Electromagnetism become observable through their properties relative to material objects in the former and specific metals in the latter, Ki becomes observable specifically in living matter.
Ki is ordinary. That is, it is in every bit of living matter. It is the natural force that through its connectivity binds inert compounds into reproducing organisms.
All of the energetic expression of the universe starts with the big bang. In the space time continuum the big bang is happening right now as well as 13 billion years ago, but it is the radiant flow of all that energy that shows up as the cosmos.
Ki is expanding from the big bang as well. As the universe spreads itself far and wide, ki churns through it all, evolving living matter to fit the whole, subject to all the forces of nature but given life by the constant flow of this energy from its source.
We can observe this energy to be connective. As previously mentioned it binds inert compounds together to form organisms. It connects organisms together to make more organisms. It is connective between predator and prey. As this energy moves through space it gathers the inert compounds in the form of bodies that born of connection, grow, mature, age and become inert compounds again.
As our ancestors developed a neo cortex they became aware of the energy they could feel inside them making them different from the ground around them. Intuitively they understood that this energy was giving them life and without it they would go back into the ground, and though they could not explain it and could scarcely describe it, they called it, for lack of another word, "god."
The life-giving and connective properties of ki are tied together but also act independently. The life-giving aspects of ki happen automatically within living systems. The connective properties, outside of the ones chemical in nature, are expressed through intention.
Through intention we regulate the flow of ki through our bodies so that we can take action. Some actions are involuntary, but all are regulated through intention. For example, we swallow automatically, but the intention of the involuntary act is to push food and liquids through the esophagus without entering the trachea. In order to get up from a chair to get something to swallow requires that intention.
Through the mechanism of intention, we release flow through our body parts to take action. But the connective properties of this energy are also regulated through intention.
The connective properties of ki flow through us to other animate and inanimate objects. Our ancestors with their newly developed neo cortex became aware of this flow of energy between them, and though they could not explain it and could scarcely describe it, they called it, for lack of another word, "love."
The reason love feels so good is because when you love you are opening channels of this energy flow to do what it does best -- connect -- and as it is literally life-giving, it feels the best when the flow is not constricted. When we love, our systems become open conduits for this connective energy. There is a reason why perhaps the most intense physical pleasure our bodies are capable of comes from the deepest connection two people can have, and that's not a connection that is solely physical.
But because we have intention as a regulating device, we can withhold our connection from anyone or anything we choose. Unconditional love is said to be the highest form, and with it comes optimum ki flow and free expression of ki's connectivity. But as we start to withhold our love, usually out of a perception of threat, we put up barriers to that flow, our intentional "floodgates".
Putting up barriers to flow to thwart the connective properties of ki to protect the self do not stop those connective properties from needing to do what they do, and as the need to connect grows within a person the ki starts to come out constricted by fear. Similar to fluid dynamics, the ki that comes out in a constricted flow, comes out like water through a fire hose and with it a destructive connection.
In the book Aikido by Kisshomaru Ueshiba under the direction of Osensei, in the section under "Basic Knowledge" after posture, distance from one's partner, and the hand blade, comes ki no nagare, which is translated "stream of spirit." Your "stream" and your attacker's "stream" merge (harmonize). In the book it is put this way: It is the work of spirit power to involve the opponent's movement into your movement, responding naturally and unconsciously to the changes of circumstances.
Two pages later in the book, after entering and spherical movement, chikara no dashikata (extension of power). The description distinguishes between force power and spiritual power and describes releasing this spiritual power through the arms hands and fingertips.
The connective and live-giving properties of ki are the essential ingredient in aikido. Our practice is to transcend ancient lower brain defense mechanisms to release the unrestricted flow of ki of which we are capable. When we do this it becomes obvious why Osensei continually called aikido a budo of love.