This chapter of Mishnah Peah defines what constitutes a 'forgotten sheaf' — how many sheaves must be left, what is considered forgotten versus intentionally left, and various cases that determine whether the owner has mentally 'forgotten' the produce.
בֵּית שַׁמַּאי אוֹמְרִים, הֶבְקֵר לָעֲנִיִּים, הֶבְקֵר. וּבֵית הִלֵּל אוֹמְרִים, אֵינוֹ הֶפְקֵר, עַד שֶׁיֻּפְקַר אַף לָעֲשִׁירִים, כַּשְּׁמִטָּה. כָּל עָמְרֵי הַשָּׂדֶה שֶׁל קַב קַב וְאֶחָד שֶׁל אַרְבַּעַת קַבִּין וּשְׁכָחוֹ, בֵּית שַׁמַּאי אוֹמְרִים, אֵינוֹ שִׁכְחָה. וּבֵית הִלֵּל אוֹמְרִים, שִׁכְחָה: הָעֹמֶר שֶׁהוּא סָמוּךְ לַגָּפָה וְלַגָּדִישׁ, לַבָּקָר וְלַכֵּלִים, וּשְׁכָחוֹ, בֵּית שַׁמַּאי אוֹמְרִים, אֵינוֹ שִׁכְחָה. וּבֵית הִלֵּל אוֹמְרִים, שִׁכְחָה: רָאשֵׁי שׁוּרוֹת, הָעֹמֶר שֶׁכְּנֶגְדּוֹ מוֹכִיחַ. הָעֹמֶר שֶׁהֶחֱזִיק בּוֹ לְהוֹלִיכוֹ אֶל הָעִיר, וּשְׁכָחוֹ, מוֹדִים שֶׁאֵינוֹ שִׁכְחָה: וְאֵלּוּ הֵן רָאשֵׁי שׁוּרוֹת. שְׁנַיִם שֶׁהִתְחִילוּ מֵאֶמְצַע הַשּׁוּרָה, זֶה פָּנָיו לַצָּפוֹן וְזֶה פָּנָיו לַדָּרוֹם, וְשָׁכְחוּ לִפְנֵיהֶם וּלְאַחֲרֵיהֶם, אֶת שֶׁלִּפְנֵיהֶם שִׁכְחָה, וְאֶת שֶׁלְּאַחֲרֵיהֶם אֵינוֹ שִׁכְחָה. יָחִיד שֶׁהִתְחִיל מֵרֹאשׁ הַשּׁוּרָה, וְשָׁכַח לְפָנָיו וּלְאַחֲרָיו, שֶׁלְּפָנָיו אֵינוֹ שִׁכְחָה, וְשֶׁלְּאַחֲרָיו שִׁכְחָה, מִפְּנֵי שֶׁהוּא בְּבַל תָּשׁוּב (דברים כד). זֶה הַכְּלָל, כָּל שֶׁהוּא בְּבַל תָּשׁוּב, שִׁכְחָה. וְשֶׁאֵינוֹ בְּבַל תָּשׁוּב, אֵינוֹ שִׁכְחָה: שְׁנֵי עֳמָרִים, שִׁכְחָה, וּשְׁלשָׁה אֵינָן שִׁכְחָה. שְׁנֵי צִבּוּרֵי זֵיתִים וְחָרוּבִין, שִׁכְחָה, וּשְׁלשָׁה אֵינָן שִׁכְחָה. שְׁנֵי הוּצְנֵי פִשְׁתָּן, שִׁכְחָה, וּשְׁלשָׁה אֵינָן שִׁכְחָה. שְׁנֵי גַרְגְּרִים, פֶּרֶט, וּשְׁלשָׁה אֵינָן פֶּרֶט. שְׁנֵי שִׁבֳּלִים, לֶקֶט, וּשְׁלֹשָׁה אֵינָן לֶקֶט. אֵלּוּ כְּדִבְרֵי בֵית הִלֵּל. וְעַל כֻּלָּן בֵּית שַׁמַּאי אוֹמְרִים, שְׁלשָׁה, לָעֲנִיִּים, וְאַרְבָּעָה, לְבַעַל הַבָּיִת:
Bet Shammai says: [That which is] made ownerless only in regard to the poor is indeed ownerless. But Bet Hillel says: it is not ownerless unless ownership is renounced even for the rich, as in the case of the sabbatical year. [If] all of the sheaves in a field are a kav each, and one is four kavs and that one is forgotten: Bet Shammai says: it is not considered forgotten. But Bet Hillel says: it is considered forgotten. A sheaf left near a stone fence, or near a stack [of grain] or near oxen, or near equipment: Bet Shammai says: it is not considered “forgotten”; Bet Hillel says: it is considered “forgotten.” [With regard to sheaves forgotten] at the end of the row, the sheaf lying across from it proves [that the first sheaf has not been forgotten.] [As for] a sheaf that [the owner] took to bring it to the city and forgot it, all agree that it is not considered a “forgotten sheaf.” These are to be considered ends of the rows:If two men begin [to gather] from the middle of the row, one facing north and the other south and they forget [some sheaves] in front of them and behind them, those left in front of them are “forgotten,” but those left behind them are not “forgotten.” If an individual begins from the end of the row and he forgets [some sheaves] in front of him and behind him, those in front of him are not “forgotten”, whereas those behind him are “forgotten,” for this comes under the category of “you shall not go back [to retrieve it].” This is the general rule: anything that can be said to fall under the law “you shall not go back” is considered “forgotten,” but that to which the principle of “you shall not go back” cannot be applied is not considered “forgotten.” Two sheaves [left lying together] are “forgotten,” but three are not “forgotten.” Two bundles of olives or carobs are “forgotten” but three are not “forgotten.” Two flax-stalks are “forgotten”, but three are not “forgotten”. Two grapes are considered “grape gleanings,” but three are not “grape gleanings.” Two ears of grain are deemed “gleanings,” but three are not gleanings.” All these [rulings] are according to Bet Hillel. And concerning them all Bet Shammai says that three [that are left] belong to the poor, and four belong to the owner.