One of the confusing aspects of purity in Judaism is that the same word, ‘unclean’ (tame') is used in two (at least) very different contexts. One is the context of animals forbidden for food, and the other of ritual impurity, which requires purification but is not in itself a forbidden state. Though quite distinct, these two areas sometimes impinge on one another.

In ritual purity, the first important principle is that live animals (with one exception, the Scapegoat) are never a source of impurity. Only animal carcases can be a source of impurity, whether the animal is permitted for food or not. A live pig, for example, is totally ‘clean’ in the ritual purity sense. To touch one does not produce impurity.

In the case of dead animals, however, the two senses of ‘unclean’ already impinge on one another: for the carcase of an ‘unclean’ animal (in the sense of ‘forbidden for food’) produces ‘uncleanness’ (in the sense of ‘ritual impurity’) more readily than the carcase of a ‘clean’ (permitted for food) animal, the latter causing impurity only if it dies of natural causes.

The main text is Lev. 11:8: ‘Of their flesh shall ye not eat, and their carcase shall ye not touch; they are unclean to you.’ Other important texts are Lev. 11: 24–28 (unclean animals); Lev. 11:29–38 (‘creeping things’); Lev. 11:39–40 (clean animals that die).

An important preliminary question emerges: ‘How literally must Lev. 11:8 be taken when it says,

’ … Their carcase shall ye not touch'?’ Here we find what seems to be a total conflict between Scripture and the rabbis.