Latest / Matmonim: Daf Yomi by Rabbi David Lapin
Chullin 123a Purposelessness (Part 1) - כיון שנקרע רובו טהורה (א)
Tumah occurs when the purpose of a human or an object to which he has given purpose, loses its capacity for purpose. Tumah contaminates the purpose (tzurah) of an object, not its sunstance (chommer).
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