Taharos, Chapter 9, Mishnah 2. This chapter takes us into the olive harvest and the oil press. A food only becomes capable of contracting tumah once it has been wetted by a liquid, and olives carry their own moisture with them: the sap that they exude as they sit and soften. But moisture only creates hechsher, susceptibility to tumah, when the owner is pleased to have it there. A liquid that arrives against a man's will, that he would rather not have at all, does not render his produce susceptible. Our Mishnah applies that principle to a man whose olives are already piled in the vat, but whose work is not yet complete.
"Gamar mililkot aval asid likach": he has finished picking his own olives, yet he still plans to buy more. The pile in the vat is not the finished pile. He is waiting to add to it, and until then the sap seeping out of those olives is not something he wants. "Gamar milikach aval asid lilvos": the same is true of a man who has finished buying olives but still intends to borrow some from a neighbor to complete the batch. In both cases his labor is unfinished by his own plan, and the moisture gathering in the vat gives him no satisfaction.
The Mishnah continues with a second kind of delay, one that comes from outside him entirely. "Ar'o evel": mourning came upon him, r'l, and he must leave his olives and sit with his dead. "O mishteh": or a celebration came his way and he is occupied with the feast. "O ones": or some circumstance beyond his control has tied his hands and he simply cannot get to the work. Here too the olives sit and sweat in the vat, but the man himself is not standing over them with pleasure. The interruption was forced on him.
The result is striking. "Afilu zavim v'zavos mehalchim aleihen, tehorin": even if zavim and zavos, people who carry a severe tumah in their very bodies, walk over that heap of olives, the olives stay tahor. Since the sap on them is moisture that the owner does not desire, it never rendered them susceptible in the first place, and produce that was never made susceptible does not become tameh no matter what touches it.
"Naflu aleihen mashkin tme'in": if tameh liquids fell on the pile, the ruling is limited. "Ein tameh ella mekom magan": only the spot that the liquid actually touched becomes tameh. The tumah does not spread through the whole heap, because the rest of the olives were never brought into a state of susceptibility.
And finally, "vehamohel hayotzei mehen tahor": the sap that oozes out of these olives is itself tahor. It is not a liquid that its owner is happy to see, so it is not treated as a liquid that carries or receives tumah.
The lesson of the Mishnah is that tumah follows a person's will. As long as the man's work is incomplete, whether by his own calculation or because life has interrupted him, the sap in his vat is an unwanted seepage rather than a liquid, and his olives remain untouched by tumah.