In the course of its continuing discussion of ritual purity (tuma v’tahara), the Gemara discusses the question of a needle that is found in the flesh of one of the sacrifices in the Temple. Rav explains that the case is when the needle may have been in direct contact with a dead body, giving it the unique status of the dead body itself, based on the rule of herev harei hu ke-halal – that a sword is considered to be like a corpse. This rule is learned from the passage in Bamidbar (19:16), which teaches that someone who comes in contact be-halal herev – with someone killed by a sword – becomes ritually defiled.
There are a number of opinions regarding this rule. Rabbenu Tam understands that an earthenware vessel that touches a corpse becomes a rishon le-tuma; other utensils (e.g. wooden ones) would become an av ha-tuma; metal utensils are unique and become an avi avot ha-tuma (see Pesahim 14 for an explanation of these terms). Rabbenu Hananel quotes Geonim as saying that the unique status of a metal object as an avi avot ha-tuma only applies if that object was the murder weapon that killed the person.
With regard to our case of the needle found in a Temple sacrifice, Rav Ashi introduces a well-known rule about safek tuma (a situation of questionable ritual defilement) – in a reshut ha-rabim (a public domain) it is considered tahor (ritually pure); in a reshut ha-yahid (a private domain) it is considered tameh. Rav Ashi concludes that in a public domain, questionable issues of tuma ve-tahara will be ruled to be tahor.
These rules of safek tuma are derived by the sages from the case of Sotah (see Bamidbar 5) – a woman who is suspected of committing adultery. In the words of the Torah we are not sure whether she has become tameh (defiled). The Torah takes the questionable situation of Sotah very seriously, and the sages learn from this that every case of tumah that is similar to such a case, that is to say, all cases that are similar to Sotah in that they take place out of the sight of the general public, should be treated stringently. At the same time, we also conclude from this that situations that are not similar – specifically cases that take place in a public forum – should be judged leniently unless proven otherwise.