Sanhedrin 48 - Deriving Benefit from a Corpse
It is certainly forbidden to use a corpse for anything, or to derive benefit from its accessory items. When does this prohibition go into effect? Abaye says that the prohibition applies from the moment the item is designated to serve the needs of the corpse, but Rava maintains that it starts from the time that item is actually used in its new capacity.
Abaye derives his view by comparison with the decapitated heifer : just as the heifer becomes forbidden when it is led into a rocky uncultivated valley, so too, all designated items are already viewed as if serving their purpose. However, Rava compares this with accessories to idolatry, which do not become forbidden through mere designation. What is Rava's reason? - We should derive accessory (for the corpse) from accessory (for idol worship), and not compare to heifer, which is the item in question itself.
Art: Howard Pyle- Dead men tell no tales