Keilim, Chapter 26, Mishnah 7. This chapter deals largely with leather items and with the question of when they enter the world of tum'ah and taharah. Earlier in the chapter we were taught a striking principle: an object that is not yet susceptible to tum'ah can become susceptible through machashava alone. That is, the owner simply changes his mind and decides that from now on he intends to use the item, and with that decision the item becomes capable of contracting tum'ah. No physical act is required, no cutting, no shaping, nothing at all is done to the object itself. Our mishnah now returns to that halachah and defines its limits precisely: when is thought sufficient, and when is thought not enough?
The Mishnah opens with the rule in the affirmative: "Kol makom she'ein chisron melachah, machashavah metame'atan." Wherever there is no missing work, thought makes them tamei. Meaning, whenever an object's manufacture is finished, when it already stands ready to serve a particular function and has a purpose it can fill exactly as it is, then the owner's thought alone is enough to bring it into the category of a keli that can become tamei. He decides that he will use it for the purpose for which it is already fit, and from that moment it is susceptible. Nothing needs to be added or altered.
Take a piece of leather that is fully suitable to serve as a mat. Until now the owner has not used it that way, and it therefore stands outside the laws of tum'ah. He then reconsiders and resolves that from here onward this leather will be his mat. That resolution by itself accomplishes everything: the leather is now susceptible to tum'ah, because physically it lacked nothing to begin with.
The Mishnah then states the reverse: "V'chol makom sheyesh chisron melachah, ein machashavah metame'atan." Wherever work is still missing, thought does not make them tamei. Here the object is not yet physically capable of doing the job the owner has in mind for it. In such a case his intention, however firm, cannot create susceptibility to tum'ah. The item remains outside the system until the remaining work is actually done and the keli is genuinely complete.
So if a man has leather in his possession and decides that he will fashion it into shoes, the leather does not become susceptible to tum'ah on the strength of that decision, because as leather it cannot yet function as a shoe. Only once the shoes are actually made does susceptibility begin. Yet notice an important point: at that stage he does not need to form a fresh intention. His original designation stands and waits. The moment the object physically arrives where it needs to be, that earlier thought takes effect and the shoes are immediately capable of becoming tamei.
The Mishnah ends by carving out a single exception: "ela ha'utzba," apart from the utzba, which is the leather covering laid over a saddle. With this item, remarkably, machashavah takes effect even while the leather still awaits its trimming and shaping. As soon as its owner designates it for that role on the saddle, it becomes susceptible to tum'ah immediately, even though the usual finishing labor has not yet been carried out.
Why is this item different? Because people are simply not makpid, not particular, about how a saddle covering looks. An untrimmed, unfinished piece of leather serves them perfectly well for this purpose, and they will readily use it exactly as it is. When the normal manner of use does not depend on that final processing, the missing work is not considered a chisron melachah at all. The item is already usable in practice, and therefore machashavah alone is enough to bring it into the laws of tum'ah.
The lesson that emerges is a beautiful one about how the halachah measures a keli. Susceptibility to tum'ah does not follow the craftsman's checklist but the reality of human use. Where an object still cannot do the job, intention alone cannot bridge the gap; but where people are content to use the object as it stands, the halachah recognizes it as a finished keli, and a person's thought is enough to make it so.