Keilim, Chapter 14, Mishnah 2. The chapter has been dealing with metal kelim, and until now the discussion revolved around a metal keli at the end of its life, one that broke, and how much of it must remain for it to still count as a keli. Rabbi Akiva now turns our attention to the other end of a metal keli's life: the keli that is newly made and not yet finished.
The principle behind the halacha
We have a rule that a metal keli in the process of being manufactured is not susceptible to tumah until its making is complete. Until the craftsman has finished his work, there is no keli here yet, and something that is not yet a keli cannot become tamei. The question, then, is what exactly counts as "finished." Which stages are part of making the keli, and which are extras that can be missing without the keli being considered incomplete?
The words of Rabbi Akiva
Rabbi Akiva's statement here comes in two short clauses. Regarding the first he rules "hamechusar chafifah tamei": an object still missing its chafifah is able to receive tumah. Regarding the second he rules "vehamechusar letishah tahor": an object still missing its letishah stays outside the reach of tumah.
A metal keli that lacks its chafifah, its covering, is nevertheless susceptible to tumah. Since a cover is not an integral part of the keli itself, the keli is regarded as complete and finished without it. The absence of the cover does not leave the keli unfinished, and therefore it can become tamei.
But a keli that lacks letishah, that has not yet been smoothed and polished, is tahor. Smoothing is part of the making of the keli itself, so as long as that stage has not been carried out the keli is still incomplete. It is not yet a keli, and so it cannot become tamei.
The point to take with us
Rabbi Akiva is drawing a line between finishing touches that belong to the keli's own body and accessories that stand outside it. Smoothing the metal is the craftsman still working on the keli; providing a cover is providing the keli with something additional. That line determines the exact moment at which a new metal keli enters the world of tumah and taharah.