Keilim, Chapter 26, Mishnah 8. The chapter has been dealing with the question of when leather goods become subject to tumah, and the principle established just before this mishnah is that an object does not always need to be finished or physically altered in order to receive tumah. If a piece of leather is already fit, as it stands, to serve a certain purpose, all that is missing is a person's decision to use it that way. Once he intends it for that use, the item is ready and can contract tumah. Our mishnah takes that principle and asks a sharper question: whose intention has this power? Not every owner's thought is treated as a final decision, and not everyone holding an object is truly its owner.
The householder and the tanner
"Orot ba'al habayit machshavah metam'atan": the hides belonging to a private householder become susceptible to tumah through his intention alone. This is a person who is not in the leather business. The skins are simply in his house and he has a use in mind for them. If he decides that a particular hide will serve him as a mat, and the hide is already suitable for that purpose as it is, nothing further needs to be done. His decision finalizes the object, and from that moment it can become tameh.
"Veshel abadan ein machshavah metam'atan": leather in the hands of a tanner, a man whose trade is preparing and marketing skins, is not made susceptible by what he has in mind for it. Suppose he sets one skin aside intending it for use in his own home, and suppose it is already entirely fit for that use: the designation still carries no weight. The reason lies in his occupation. A dealer sells, and tomorrow he may reconsider and pass that very skin to a customer, who will want something else from it altogether, shoes perhaps, and shoes demand cutting and stitching before the article counts as complete. Leather still awaiting such work is not yet susceptible to tumah. Because that scenario always remains open, we never treat the tanner's thought as a closed decision. He has performed no act upon the skin, and the wishes of whoever ends up using it are unknown to us.
Of course, if the tanner actually works the leather rather than merely thinking about it, the picture changes entirely. Once he has fashioned it into a pair of shoes, no further processing is expected of it, and it is fully subject to tumah like any finished article.
The thief and the robber
The mishnah now turns to hides that have been taken unlawfully, and here we need to distinguish between two kinds of criminals. A gannav steals furtively, without the owner's knowledge, breaking in when nobody sees. A gazlan takes property out in the open, by force or by threat, a strongman who robs in broad daylight and is not afraid of being recognized.
"Shel gannav machshavah metam'atan": if the hides were taken by a furtive thief, his intention does render them susceptible. Should he decide to use them just as they are, they can immediately contract tumah. The reason is that the theft was done in secret and the owner has no idea who took his property. Having no realistic way of tracing it, he despairs of ever seeing it again, and that ye'ush transfers ownership to the thief. He has certainly transgressed the Torah and acted wickedly, but halachically the hides are now his, and therefore his thought about them determines their status.
"Veshel gazlan ein machshavah metam'atan": where skins were seized by an open robber, the ruling is the reverse, and his intention accomplishes nothing. This robbery took place in full view, and everyone knows exactly who committed it. The victim can drag him before Beit Din, he can call in the authorities, and he will press every avenue available to get his property back. Since no despair has set in, ownership never passes to the robber at all, so whatever he plans for those skins leaves their halachic status untouched.
The view of Ribbi Shimon
"Ribbi Shimon omer chiluf hadevarim": the two cases, he holds, must be exchanged. "Shel gazlan machshavah metam'atan": in this view it is precisely the skins of the open robber that intention renders susceptible. Why would a man plunder in broad daylight without troubling to conceal his face? Because he is confident that no court can touch him. He has the upper hand; there is nobody to indict him and nobody able to enforce a judgment against him. The victim reads the situation for what it is and writes his property off completely. That despair transfers the skins to the robber, and once they belong to him, his plan for them fixes their status.
"Veshel gannav ein machshavah metam'atan mipenei shelo nityashu habe'alim": the skins of the stealthy thief, by contrast, lie beyond the reach of his intention, and the mishnah supplies the reason: their owners never gave up hope. A man who creeps into a house under cover of darkness is himself frightened, nervous that the authorities are already on his trail. And the victim, even without knowing who entered his house, has solid grounds to expect that inquiries will turn up a lead, that the guilty party will be identified and compelled to return what he took. With hope still alive, the thief acquires nothing, and no thought of his can affect those skins in the slightest.
The mishnah thus teaches two boundaries on the power of machshavah. Intention finalizes an object only when the person who thinks it is genuinely settled in his plan, which the merchant tanner is not, and only when the person who thinks it is genuinely the owner, which a thief or robber is only in the case where the victim has despaired of recovery.