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Dear Ian,
It is difficult for any of us to say those things that we think are important in a compact form. I will give it a try:
I think that the point at which math starts to diverge is when we introduce indefinable units. However, let me say this in a different way. Returning to f=ma (I purposefully do not begin with f=dP/dT), the only part of this equation that can be explained is acceleration. That is because it consists of measurements of distance and time. The rest of the equation is unexplained. Force is the unexplained cause for acceleration. Mass is the unexplained cause for the variations of that acceleration. Symbols and names of convenience are used to to form this equation. This practice is helpful without causing problems.
The properties involved are all empirical properties. Two of them are distance and time. These are real directly known indefinable properties. The other two properties are force and resistance to force. These two are indirect empirical properties. Both force and resistance to force have unobservable natures. We know from empirical evidence that they exist; however, all of that data consists of measurements of distance and time. Measurements of distance and time are the extent of direct empirical evidence.
All higher level theory will consist of combinations of these four properties. Everything else added on changing this simple perspective is theoretical. The distortional effects of theory are first introduced by chosing to define mass as an indefinable property requiring its own indefinable units. From this point on all theory using this definition of mass is no more correct than is that first definition. It is important then to get the definition of mass correct right from the start.
I have described mass as the cause of resistance to force. We do not know the nature of any cause. Cause is hidden from us except through its effects. I think that the best approach to reconsidering the definition of mass is to add nothing on to the equation and rely only upon our use of empirical evidence in the form of distance and time and their units only. That is the concept that I used in my earlier message where I showed one possible new way to interpret mass. I have gone a long way by trying out this approach. Perhaps I may have gone astray myself; however, what I can say for certain is that: This idea does not quickly fall apart, it just keeps on going and going.
I will stop and see what you think.
James