Re: to ki or not to ki
At this point, there is no instrument for measuring ki: nothing analogous to a barometer or thermometer or scale. The only measurements of ki are from human beings, self-reported, subjective and unverifiable. The phenomenon called "ki" may exist, but to say that you "know" it exists because your subjective sense perceives something that you choose to label in such a way, and that no outside source can corroborate, is a bit humpty-dumptying the definition of "know", IMO. Perhaps it's better to say that you believe it exists because you've felt something that you lack another explanation for. The problem with such beliefs is that they can become a little too firm, to the point of refusing to consider alternate explanations that are readily available.
I'm a ki agnostic, myself. I don't know if it exists; I think there's a good chance that whatever it is may well turn out to not be some mystical force after all, but simply the product of many different factors of physics and physiology, and that combine in ways so complex that they haven't yet been explained. Please note that there's a big difference between "haven't yet been explained" and "can't be explained": there are many natural phenomena that haven't yet been explained, simply because no one has expended the time and energy and resources to do so, and others that haven't been explained because of lack of the tools to fully explore them. Mars was red before we had the means to acquire evidence to explain why; the cause of its redness hasn't changed simply because we sent the Rover.
As for aikido, I don't believe in mystifying it. Every day, in and out of the dojo, I encounter phenomena for which I have no neat and tidy explanation. My reaction is not to attribute them to some mystical force, but instead to gaps in my own knowledge. I believe that these gaps will always exist, and that the quest for some "unified field theory" of aikido is doomed to failure: as soon as you think you've nailed the neat and tidy explanation that sums it all up, someone's gonna do something that doesn't fit your theory. If you need a metaphor or an explanation for what you experience in the dojo, I suppose ki does as well as anything. My only concern is its potential for becoming an "emperor's suit of clothes", where people become convinced that they have to believe in the existence of this phenomenon, and are persuaded to claim that they experience it, and to make pronouncements about its nature and where it is and isn't found, just to avoid being labeled as dolts who just don't get it. I have no problem saying that I don't see the emperor's clothes; they may exist, and I may see them some day, but I don't see them now, and I don't feel that I'm somehow not getting it if I never see them.