Gandalfs Beard wrote:
I was just meaning that if you actually adapted a film from the books yourself, that as Pure as you would try and make it, you would invariably find people even more Purist challenging your vision

. It's all relative

.
Certainly there is no objective, universal standard that one can use for purism, but neither is there one for "heathenism" (as Odo calls it

), or revisionism if you prefer. Purists will of course disagree over what exactly constitutes a fundamental part of the story (I for one see no problem removing Bombadil, while others see it as major issue). Purists can at least agree however, for the most part, about method for adaptation (i.e., preserve the fundamental story - including characters and plot - even if you change the way in which it is told).
As I mentioned above though, the alternative is not any more objective. It is largely based on an idea of the "spirit" of the book. At least for purism there is an objective text that can be referred to rather than personal interpretations. Other variations of revisionism base changes on the need to make the story easier to follow or appeal to modern audiences, but again these are personal and far more relative than differing interpretations of which parts of the text are most important.
We can still debate the merits of each method of adaptation though, even with the knowledge that neither gives an exact guide of what to do for a scriptwriter.