Makhshirin, Chapter Six, Mishnah Two
1) One who carries up to the roof bundles [of vegetables] or cakes of figs or garlic so as to keep them fresh, it does not come under the law of if water be put.
2) All bundles [of vegetables] in the market places are unclean.
a) Rabbi Judah declares them clean if they are fresh.
b) Rabbi Meir said: Why did they declare them unclean? Only because of liquid from the mouth.
3) All coarse and fine flours of the market places are unclean.
4) Crushed wheat, groats, and pearl-barley are unclean everywhere.
Section one: Since he carried his vegetables or other such items up to the roof only to let them air out and not so that they would get wet, they do not become susceptible to impurity if dew falls on them.
Section two: The assumption is that the sellers in the marketplace moisten their vegetables to keep them fresh and make them look more appetizing (supermarkets still do this with their mist machines). In the marketplace impure people might touch them. Therefore, when one buys vegetables from the market, one has to assume that they are impure.
Rabbi Judah says that sellers don’t throw water over fresh vegetables, so they are considered clean because they were never made susceptible to impurity.
Rabbi Meir disagrees as to why they declared bundles of vegetables to be unclean. Bundles come untied and when they do a person uses his mouth to moisten the ties so they can be retied. The assumption is that the person who does this is impure and this moisture from the mouth is what causes them to be defiled. So even fresh ones, ones that have never had water put on them, are impure.
Section three: The wheat used to make flour is moistened before it is ground up. This moistening causes it to be susceptible to impurity. In the marketplace we have to assume that anyone might have touched the flour, including impure people. Therefore, it must be assumed to be impure. Outside of the marketplace impure people won’t necessarily be touching the flour so it remains pure.
Section four: These crushed grains are frequently touched, unlike flour and fine flour which not as many people will touch. Since people touch them everywhere, not just in the market, they are always considered impure.