On daf 14, you wrote the following decription for the explanation of DAVAR HA'NADUR: "another object which has been prohibited either through a Neder or by becoming Hekdesh (consecrated), i.e. that has been prohibited because of an Isur that a person placed upon it."
In our shiur, someone raised a question as to why hatfasah does not work in avodah zarah? He postulated that when one creates an idol, that is also in effect something "which has been prohibited because of an isur that a person placed upon it," since it now has acquired its status of avodah zarah through the person's action. (I suppose the same question could be asked regarding basor b'chalav.)
I would appreciate your comments on this. I thought that the difference lies in the way an object becomes prohibited. In the case of something that is consecrated, there the person merely utters words which in turn affect the object directly. Although this in fact only works because the Torah says so, still, after the Torah has established this, it is now the person who directly creates an isur in the object.
However, when one creates an avodah zarah, the isur placed on the object is not as a direct result of the person. Rather, because of the person's actions, the Torah now says that the object is prohibited. Hatfasah in this object has no effect on another object because this object of avodah zarah was automatically prohibited by the Torah, similar to the meat of an unkosher animal.
If we learn like the shita of the Ritva, it is much simpler. He says that DAVAR HA'NADUR means an "isur cheftzah," as opposed to an "isur gavrah." He says further that every isur of the Torah is an "isur gavrah," except for neder, which is an "isur cheftzah."
As you write, the concept of Davar ha'Nadur implies something that becomes Asur exclusively through one's Dibur.