Reference for Bava Metzia 200:7

אכתי הילך הוא אמר רבא עבדא דקטע לידיה ושדה שחפר בה בורות שיחין ומערות

R. Meir said: In the case of slaves he can say to him [the owner], 'Behold, here is yours before you!'<span class="x" onmousemove="('comment',' Because slaves, like real estate, cannot be stolen, i.e., they never quit the original ownership through theft, and are considered to be, and grow old, in the legal possession of their rightful owner. This contradicts what has been stated, namely, that R. Meir treats slaves as movables. ');">⁷</span>

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