Hi Eddy,
First off, I would like to apologize for leaving off the earlier discussion with my last comment in a bit of a huff. I guess I was just frustrated that you thought I was belaboring the point when my intent was just to answer your points (though as usual, I was a bit long-winded about it

). And also about what seemed to me at the time an unjustified introduction into the discussion of the concept of notation. But I'm glad that you posted a more formal outline of your argument because now I have a better idea of it (also from looking at some of the previous portions of the thread (and Rainer's mediation also helped

). While I still don't agree with your conclusions, I at least think I have a better point of departure from which to respond.
To be perfectly clear (and to verify and for anyone else who might read the thread), I would first like to restate what I at least have interpreted as the position. You are arguing that rubato affects only tempo, not rhythm, and that this makes sense because rhythm by definition is fixed according to its notation while tempo on the other hand is fluid and changeable according to its performance and that rubato is also a performance element so it makes sense that rubato would therefore affect only tempo. And I am saying that it affects both tempo and rhythm, rhythm being a constituent part of tempo, and that this is because they are both subcategories of duration, which is overall the only element being affected by rubato if we consider that there are only two basic
performance elements that are variable in music: duration and force (dynamics) (and a third, pitch, in certain instances).
First, I would start off by commending your argumentation (which I am defining as the logical progression of the steps in the argument) itself, which IMHO is excellent -- very clear and sequentially laid out. However, there are two basic criteria for the validity of an argument (not saying you don't know this, but just laying it out to illustrate my point): it must be (1) logical and (2) sound. (1) says that the steps of the argument progress and follow, while (2) says that it is based on solid, unquestionable premises. I am convinced that you basically have fulfilled (1), but I have objections on (2). The general nature of my objection is that the premises are too narrow and restrictive. Specifically, I would object to the following:
1. Your assumption that the definition of rhythm is only fixed according to its notation. I would begin by defining notation, i.e., the system of signs and symbols, notes, their values, and otherwise, that a composer uses to write or mark his score.
I would attempt to illustrate the falseness of this by appeal to relevant analogy. First, a line of reasoning similar to that I used on that MIDI thread when I briefly argued against photography as a viable artistic medium. This is that the score in music is nothing more than the sum of its markings and means nothing until it is brought to life by a performer. That is, unlike, say, a painting, which is literally the work in itself and, once being completed by the artist, is there to be admired as the work in itself, music requires a performance and a performer(s) in order to have meaning or be appreciated. This makes music an art that, to fulfill its message as art, must always be
applied independently of the score or notation. Therefore, rhythm and meter, as one constituent part of this art, are more than just the sum of their notation; they mean nothing on the page until brought to life by the performer and thus are inherently always applied. You mention pitch as a parallel case to rhythm, arguing that pitch is always fixed as well, but I think that is wrong. On the piano, presuming it could of course ever be in perfect tune, that may be the case, but for vocalists and violinists, that is an essential technical element of their performance that can be criticized or lauded as the case may be, and there are an infinite number of slight gradations that can be brought out on a fractional scale. Frank Sinatra, in fact was known for doing all sorts of slidings and elisions down fractions of a step in some cases that were of course not notated in the score.
Second, a much briefer example. Consider situations in which we don't have a score at all, say, a group of children clapping. Some will be on the beat and have good rhythm, some will be off the beat and have questionable or poor rhythm, depending on their musical ability. It is according to a metrical beat and roughly estimated division, so it is therefore rhythm (as I think we have defined it), but we have qualitatively judged it according to their ability to maintain the beat, which is applied rhythm in performance so there is no score or notation yet they are still using rhythm.
Conclusion: Your premise involving the definition of rhythm is too narrow because it fails to consider other valid definitional aspects of rhythm besides its notation and, indeed, the most important aspect -- that it always must be applied in some sort of performance and that this can, in fact, be done without the existence of a score or without the presence of music's other aspects. I know you may say that your argument is in the context of formally notated music such as, for example, the piano music we record, but this brings me to my second objection:
2. Your use of the term "musical argument" to justify your position. This is, I believe, a commission of the logical-depth fallacy (deliberately insufficiently considered premises). That is, once its parameters have been defined (i.e., in this case whether rubato affects rhythm, tempo, or both),
any and all information, including that from other relevant disciplines, is admissible as evidence. I think you tacitly agreed to this by admitting of the possibility of the broader scientific viewpoint, but the point is that all viewpoints must be considered as part of one's own viewpoint in order for one's conclusions to be valid on the basis of the evidence. Then one must go back and incorporate that evidence if the opposing conclusions are contradictory. By way of example, Aristotle in his Ethics argues that moral excellence is based on habits acquired in one's youth through good instruction (an ethical aspect) but then also extends this to good deliberation and practical wisdom (an epistemological viewpoint). If he had said that he were only treating of the "ethical argument" he would be failing to consider vital evidence.
Overall Conclusion: Though you have successfully described one aspect (the notational aspect) of the debate, you have failed to take account of the fact that all musical elements that are written into a score will still be dependent on their musical performance to be actualized. Now to return to my argument. The fact that the rhythm in a score can never be perfectly rendered by a human being (e.g., without the use of a machine or midi program) means that rhythm can never be rendered perfectly in a musical performance, musical performance being the only way of conveying music to make it do what it does as an art. Therefore, rhythm
can, and in fact, must be affected by human performance variables. Rubato is one of those variables, which indeed affects the tempo, albeit only briefly when it is applied, but then also affects the meter or rhythm, which is a constituent part of the tempo since the tempo is an overall pace and the meter are the divisions according to which the performer keeps that pace Then both tempo and rhythm are subcategories of the larger category time or duration, which is in effect, all that is being manipulated by the performer with respect to the time element. Therefore, on the basis that the whole is the sum of its constituent parts, both rhythm and tempo are affected.
Furthermore, in closing, I would also point out (though this is not an official part of my conclusion but more of a conjecture), rubato may even affect rhythm
more than tempo if it is ideally applied (which of course is not actually possible for a mere mortal), since the purpose of rubato has always been described as not to change the overall tempo, but that in fact, when time is robbed it should be made up later, in which case the net effect on the tempo would ideally be zero, and since rubato is applied only to small fragments of a piece and not a whole piece, it would affect the smaller divisions of the piece, where the individual rhythmic elements are more the issue.
Well anyway, enough pontificating from me, I thought I would just recast my position again, since you have also put forth yours twice on this thread already

Thanks again for raising this interesting point. So anyway, I've basically said my peace and added some more verbiage to this discussion

No doubt you won't agree with me or still have objections but that's fine. Nevertheless, I always learn much from you (especially about theory aspects, which not at all an expert in), even if I don't agree and I very much enjoy such discussions and hopefully look forward to more in the future (or possibly on this thread, pending your reply).
Regards,
Joe