The following is a bit ugly and unpolished, though hopefully useful in draft form. The short essay is part of an on-going discussion between Ron Ragusa and me on AikiWeb. Most recently, we are considering the roles of uke and nage---are they two sides of the same coin?---and we are questioning whether it is necessary to "hold" the state of mushin. Here are my latest thoughts:
Before there can be "just aikido," there is "learning aikido," which generally includes principles and techniques as well as the roles of uke and nage.
In "learning aikido," it is useful to experience "completely centered" and "completely committed" as two extremes of the "placement" of attention / intention---or "ki." I might call the absence of any particular intention or any particular attention to something---such as an adversary's wrist or weapon---mushin, or "completely centered;" and, I might call the laser-like focus upon something---such as "I will grab his wrist!"---"completely committed."
I believe that one type of valuable uke-nage interaction is for uke to practice being completely committed and for nage to practice being completely centered. Practicing in this extreme way, at whatever speed, is often perceived to be ridiculous from the self-defense or martial point of view; however, from the perspective of learning what it is to have mind and body integrated, learning what it is to be led (or, to have "ki led") versus what it is to be "centered," this is very valuable training.
In never practicing this way, we may never sense the full range of the continuum of attention / intention; however, in only practicing this way, we are subject to an equally terrible error: We may develop the habit either of always yielding / being led, or always remaining firm, seeing only the extremes, unable to handle situations in between.
In training we eventually experience what one might consider a third state: distraction. Whether distraction occurs by virtue of something "real" to the encounter such as the momentary blinding by a bright light," or by virtue of something "not real" to the encounter, such as wondering what is for dinner, we should see it for what it is, something essentially no different than focusing upon your adversary's grip or trying to make that technique work, knocking you from your centered (or focused) state most unproductively. Here again, though, if we can recognize the affects of distraction upon our state of mind and body, we can learn to use distraction to our advantage as well as not to be hindered by it.
As we progress from "learning aikido" toward "just aikido," I believe the roles of uke and nage naturally (and, hopefully) tend to dissolve in advanced practice. More time for both players is spent in mushin, attachment to distractions as well as to outcomes of launched attacks and defenses is relaxed, and focused intent becomes more of an impulse rather than sustained effort. This is not to say that attacks or defenses are necessarily less powerful; rather, like shooting an arrow, we are at the ready, and when an opening is noted, the arrow is let loose with only an impulse. Like this, the body is set into motion and immediately returns to its centered state, ready to adjust the action, to deal with a counter, or to launch the next blow. Every action, then, is generated from the centered state; this is mind and body integrated without resistance. Releasing focus upon performing a particular technique---once there is physical competence with that technique, of course---as nage, and feeling when your opponent is off-balanced in performing a technique upon you as uke, this is the foundation for being sensitive to changing conditions, which in turn is the root of changing technique (henka waza) and reversal (kaeshi waza) practices on the mats---as well as the prevention of the same.
Consider an example: I am not persisting with the Ikkyo technique, simply because I am doing Ikkyo---I am not doing anything. I launched what you might call "Ikkyo" (if you were forced to classify it) and returned to center; the body knows the rest from practice, and it is open to what happens next through relaxed mushin. To say, "I am doing Ikkyo," or forcing an interaction to resemble a model Ikkyo, or driving on toward completing a model Ikkyo past when the interaction calls for it or past it being an appropriate response, are all actions rooted in thought, concepts, and ideas, rather than the the situation and your center. Mushin is long gone; seasoned practitioners know the resulting difference in feeling.
The question has also been raised regarding whether mushin is something to be pursued or whether it is something that appears quite naturally when necessary. The active pursuit of being centered is in itself an off-balancing activity by definition---after all, we are focused upon the activity; however, in much the way that medicines are poisons to what they cure, there are focused practices to bring about centeredness. Does mushin naturally appear when needed? Insomuch as mushin is related to that "everything was moving in slow motion"-type experience we feel under extreme circumstances, I suppose it does; however, we should consider these points:
(1) Does mushin's appearance guarantee action, let alone an appropriate action? The clarity to see something falling and to grab it---is this mushin or simply instinct? What if the falling object was a hot pan from the stove? Can we improve the appropriateness of our responses with practice?
(2) Is there value in maintaining mushin under other-than-extreme circumstances? Arguably, most people spend most of the time distracted by some combination of their own senses and their own thoughts---sleep walking, so to speak. Is there value in practicing to maintain centered awareness under more ordinary circumstances?
It is hard to imagine an answer other than "yes" to either question.
But this discourse is very cloudy in itself. Let's consider a different way of responding: If you asked me, "Are the roles of uke and nage the same or different?" and I responded, "I hate when my partner throws himself," would you appreciate the meaning now given the above?