What is Rav sheshes' Reasoning that he concludes even things that pass through a Tamei is tamei. Gezaras HaKasav is not his reason it is his support. Likewise the Mishna Min instead of m'tamei is support but not his reason why holds his position. Everyone agrees that if it is part of the tamei it is tamei. Why (what reason) does Rav Sheshes say also something that just passes through is tamei when it is not part of the tamei?
Avi Weinstein, Efrat Israel
The rule of Yotsei min ha'Tamei is metioned in the Mishnah (5b) and the Gemara (6b) learns it out of a Pasuk (according to the Rabanan from the word Gamal, and according to Rebi Shimon from the word Es of Es ha'Gamal. (The Rambam (Machalos Asuros 3:1) brings a different Pasuk.)
The logic behind this rule would appear to be that whatever is produced from something is considered as a part of the thing that produced it. This is because something that is produced is produced from the essence of the thing that produced it and as such has the same Halachah as the thing that produced it. This is the General rule, although there are some exceptions that are not considered to have been produced by the essence of the thing that produced them, and do not therefore have the same Halachah as the thing that produced them, for example urine of horses and camels according to the first Lashon of the Gemara on 7a, or honey (7b).
Dov Freedman