A vessel of earthenware can protect everything [in it from contracting impurity], according to Beth Hillel.
But Beth Shammai says: “It protects only food and liquids and [other] vessels of earthenware.” Beth Hillel said to them: “Why?” Beth Shammai said to them: “Because it is [itself] impure with respect to an ignoramus, and no impure vessel can screen [against impurity].” Beth Hillel said to them: “And did you not pronounce pure the food and liquids inside it?” Beth Shammai said to them: “When we pronounced pure the food and liquids inside it, we pronounced them pure for him [the ignoramus] only, but when you pronounced the vessel pure you pronounced it pure for yourself and for him.”
Then Beth Hillel changed their mind and taught according to the opinion of Beth Shammai.
According to Numbers 19:15, a clay vessel that is covered with a lid prevents impurity from entering inside of it. If this vessel is found in a room with a dead body, which would normally cause everything in the room to be impure, the clay vessel and all that is inside of it does not contract the impurity. Beth Shammai and Beth Hillel argue about what types of things which may be inside the clay vessels are not impure. According to Beth Hillel any object inside the vessel is pure. Beth Shammai holds that only food, liquids and other clay vessels remain pure; non-clay vessels would be impure.
Beth Shammai explains that we can assume that the clay vessel has been touched by an ignoramus (am haaretz), a person who does not strictly know or observe the laws of ritual purity. It is assumed that the am haaretz makes the vessel impure. Since an impure vessel does not prevent the impurity from entering, the things inside of it are impure.
Beth Hillel responds to Beth Shammai by pointing out that they did indeed accept that the food and liquids inside the vessel were pure. If the clay vessel does not prevent impurity from entering, why should anything inside of it remain pure?
To this question Beth Shammai responds that when they stated that the food and liquids were pure they meant for the am haaretz himself and not for the haver (a person who scrupulously observes the laws of purity and indeed eats only pure food). Beth Shammai assumes that a haver will not borrow any of these things from an am haaretz, since they cannot be made pure (a clay vessel cannot be cleansed of its impurity). Therefore Beth Shammai can pronounce these things clean, knowing that they will never come into the hands of a haver. However, when Beth Hillel pronounced everything inside pure, they were in essence declaring it pure for both the am haaretz and the haver. Beth Hillel had implied that even metal vessels, inside the clay vessel, remained pure. A haver might borrow metal vessels from an am haaretz, with the intent of immersing them to cleanse them of their impurities. However, this immersion will only cleanse them from light impurities and not from impurity contracted from a dead body. Therefore, an am haaretz might borrow them thinking that he could cleanse them and in reality he could not. Due to this problem, Beth Hillel retracted their opinion and taught like Beth Shammai.