Sanhedrin 48 - Deriving Benefit from a Corpse
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: Dead Men Tell No Tales by Howard Pyle
