Furuya Sensei posted this to his Daily Message on October 5, 2003.
I was taught mai-ai (間合い) or “spacing” early on in my training. In very traditional Aikido, one respected Aikido teacher taught that ma-ai is the distance in which we can see our opponent straight on but still see their feet and arms (lowered at their sides) in our peripheral vision. If we are standing slightly too close to our opponent, their feet and also their hands will disappear from our vision. Of course, if we are slightly too far from our opponent, it is possible to see their whole body easily, but then we are at a distance in which they are not compelled or inclined to attack and we also cannot effectively apply our technique either. At the proper distance or mai-ai in this case, the opponent does feel inclined to attack. In addition, this teacher also taught that one takes kamae in such a way as to very subtlety control how the opponent may attack us. Or briefly, positioning one's self so that they are inclined to attack us with their right as opposed to their left hand, etc. This is also a part of ma-ai, according to him.
Another teacher taught that ma-ai is the special spacing or "angle" in which you can strike at your opponent but they cannot strike you. Or, the spacing in which we are protected against our opponent's immediate counter attack while still maintaining a position to easily apply our own technique.
Another teacher taught that ma-ai is the spacing in which the technique can be executed at its optimum level. If we are too close to our opponent, we are open to their counter attack and/or the possibility of them jamming our technique. Of course, if we are too far away from our opponent, the technique cannot be applied properly at all.
Another Aikido great taught that the ma-ai must constantly be adjusted to neutralize or adjust the speed and strength of the opponent's attack by closing in or creating more distance between one's self and the opponent.
I consider all of this in my study, practice and execution in all of my techniques.