Midrash sobre Levítico 5:2

1) (Vayikra 5:2) ("Or if a soul touch any unclean thing, or the carcass of an unclean animal, or the carcass of an unclean beast, or the carcass of an unclean creeping thing, and it be hidden from him, and he is unclean and he is guilty") "Or if a soul touch any unclean thing": The early masters were wont to say: I might think that even if one touched something that had touched an unclean thing (tamei), he was liable (for entering the sanctuary or eating consecrated food); it is, therefore, (to negate this) written "the carcass of an unclean animal," "the carcass of an unclean beast," "the carcass of an unclean creeping thing" — Just as these are unique in being avoth hatumah (primary causes of tumah), (so all that are avoth hatumah are included), excluding those which are not av hatumah).

2) R. Akiva said: I might think that even if one touched food, drink, or earthenware vessels (that had become tamei) he was liable, these are excluded, not being avoth hatumah.

3) Or, if you learn a thing in one way (viz., binyan av, see Hermeneutical Principles 3), you learn it in all of its aspects — Just as an unclean beast is unique in that touching it is distinct from carrying it (i.e., by touching it, one's garments do not become tamei, whereas by carrying it they do) and it becomes an av hatumah to render tamei both man and beast, so I include only those things which are like it. What do I include? The qualifying amount of the sprinkling waters of the red heifer and the saddle (mercav) (of a zav, one who is unclean by reason of a seminal emission), the touching of which is distinct from carrying them, (touching not rendering one's garments unclean; carrying, doing so) and which become an av hatumah to render tamei both man and beast. "any unclean thing" includes the chair and the couch (of a zav), whose touching is like its carrying and which become an av hatumah to render a man tamei to make his garments tamei. "unclean" includes the burner of the red heifer, and bullocks (the bullock of the anointed high-priest, the bullock of "forgetfulness" (helem davar) of the congregation, and the Yom Kippur bullock), and the sender-away of the (Yom Kippur) scape-goat, without touching, (a man who touches them not becoming tamei). "any unclean thing" includes (for tumah liability in entering the sanctuary a man who walks under unclean) overhanging boughs, (under unclean) jutting stones (adjoining a cemetery), and (who touches) semen (even though carrying it does not confer tumah). These are the words of R. Yehudah.