Shabbos, chapter ten, mishnah 1. In the previous chapters we discussed the shiurim (minimum measures) of various items - the quantity considered significant enough to render a person liable for carrying them out and transporting them on Shabbos. Our mishnah deals with a case in which the item does not reach those measures, and nevertheless a person can become liable for it.
The words of the mishnah and their explanation:
"HaMatzniya" - one who sets aside a certain quantity of a certain item for a purpose he has designated for it.
"l'zera" - he took a seed and set it aside in order to plant it.
"l'dugmah" - he set aside something to serve him as a sample, to show it and tell about it, for publicity and the like.
"ul'refuah" - he set it aside for medicinal purposes.
"v'hotzi b'Shabbos - chayav b'chol shehu" - once he set it aside and thereby showed that he considers this quantity significant for himself, if he carried it out into the public domain on Shabbos he is liable even for the tiniest amount.
"v'chol adam ein chayav alav ela k'shiuro": Other people, who did not set the item aside and did not designate it for a particular purpose, do not become liable for such a quantity unless it reaches the measure that the Sages established in the previous chapters.
"chazar v'hichniso - ein chayav alav ela k'shiuro": If, after he set it aside and gave it importance, he changed his mind and no longer intends to use it - not for planting, not for a sample and not for medicine - and he brought it back into the house, he is not liable unless it contains the ordinary measure that the Sages established for it.
The reason for this: by bringing it back, he nullified his original intention. The original thought, that this small quantity is significant for him, has been nullified, and the item is now like everything else. Therefore, when he brings the item back into a private domain, he becomes liable only for the ordinary measure, and not for the measure he had set for himself beforehand.
In summary: We have learned that a person's intention establishes a measure for him personally - one who sets something aside for planting, for a sample or for medicine is liable for carrying it out in any amount at all, whereas other people are liable only for the measure established by the Sages. And just as the intention grants importance, so its nullification removes it: once he brought the item back in, it returns to its ordinary measure.