Reference for Niddah 70:5

ותסברא להחמיר עליו שהוא עושה משכב ומושב בראייה ראשונה בר משכב ומושב הוא

after a first observation to cause a couch and a seat to be unclean? But the fact is that it is this that was meant: 'R. Isaac said, A <i>zab</i> after his first observation was surely included in the same law of uncleanness as one who emitted semen, why then was he in the case of a second observation excluded? In order to relax the law for him in one respect and to restrict it for him in another respect. "To relax the law for him" in that he does not become unclean in case of a mishap; "and to restrict it for him" in that he causes a couch and a bed to be unclean'.<span class="x" onmousemove="('comment',' As a 'father of uncleanness'. ');">⁵</span>

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