Reference for Chullin 201:2

ור"ש מה נפשך

But whatever you think is the opinion of R'Simeon [there is always a difficulty]! If he holds that one prohibition can be superimposed upon a pre-existing prohibition, then he should have incurred guilt on account of the nerve too; and if he holds that one prohibition cannot be superimposed upon a pre-existing prohibition, then he should have incurred guilt on account of uncleanness, for that came first;<span class="x" onmousemove="('comment',' Namely, while it was still an embryo in the womb before the formation of the nerves; v. supra n. 1.');">¹</span>

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