Now we can explain the Rambam we questioned earlier. As you may recall we noted that in halacha aleph the Rambam implies that one is only a Navi Sheker if he tries to change what is written in the Torah. But then in halacha dalet the Rambam says something completely different, namely that one can be a Navi Sheker even when he doesn't claim that the Torah changed, just that he was told from שמים that the halacha is a certain way. In such an instance he is also considered a false prophet - against the implication of halacha aleph.
The answer is as follows: The concept לא בשמים היא means that halacha must be decided based on the 13 middos and the like and not on any heavenly inspiration or revelation. In halacha aleph, the Rambam is talking about adding on to or subtracting from the Torah or saying that mitzvos are only temporary. That has nothing to do with the Torah being given over to the Chachomim and לא בשמים היא. On the contrary, if they would be able to add or subtract that would have to come from שמים. The Chachomim only have power over what they were already given but not beyond that. So if a person tries to add to or subract from the Torah he is killed as a Navi Sheker, which is what the Rambam writes in halacha aleph when he says that the Torah can never change and by claiming otherwise he is a false prophet.
In halacha dalet the Rambam is talking about something completely different. There the Rambam is talking about deciding the halacha based on Divine revelation which flies in the face of the rule לא בשמים היא. So the question [asked by the Kesef Mishna] that halacha aleph implies that one can only be a Navi Sheker if he rules against the Torah [adds or subtracts etc.] and halacha dalet says that even if one doesn't pasken against the Torah he is a Navi Sheker, is not difficult. Because in halacha dalet the Rambam is talking about a Navi Sheker as well but one with a completely different מחייב. The halacha dalet Navi Sheker is due to his contravening the rule לא בשמים היא while the halacha aleph Navi Sheker is because he falsified the Torah. Hence, they were split up into two separate, distinct halachos and categories.