Mischna zu Wajikra 15:5

Rabbi Joshua and Rabbi Nehunia ben Elinathan, a man of Kefar Habavli, testified concerning a limb [separated] from a corpse that it is impure; whereas Rabbi Eliezer says: they declared [this] only of a limb from a living [man]. They said to him: is not there an inference from the minor to the major (kal vehomer): If in the case of a living man [who is himself pure] a limb severed from him is impure, how much more in the case of a corpse [which is itself impure] should a limb severed from it be impure! He said to them: they have [nevertheless] declared it only of a limb from a living man. Another answer: The impurity of living men is greater than the impurity of corpses, because a living man causes that on which he lies and sits to become capable of making impure a man and clothing, and [he causes also] what is over him to transfer impurity to foods and liquids- which is defilement that a corpse does not cause.