Reference for Shabbat 117:4

ר' אלעזר אומר במדרסות אומרים עמוד ונעשה מלאכתנו ואין אומרים בטמא מת עמוד ונעשה מלאכתנו ור' יוחנן אמר אף אומר בטמא מת עמוד ונעשה מלאכתנו

and sits upon it, or a <i>tarkab</i><span class="x" onmousemove="('comment',' Half a se'ah. ');">⁶</span> and sits upon it, it is unclean: hence it is stated, 'whereon he sitteth', teaching, [only] that which is appointed for sitting, excluding this, where we say to him, 'Get up, that we may do our business!'<span class="x" onmousemove="('comment',' I.e., the zab would be told that the measure is needed for its main purpose; hence it is not unclean. This shows that as a general principle every article is regarded from the point of view of its original and primary function. ');">⁷</span>

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