Pesachim 62 - How to use 'since'

We have seen the principle of since again and again, but now Rav Chisda is using it in a novel way. The Passover offering must be brought, having in mind the people who can eat it, who have signed up for it, and who are circumcised. What if he brought it for someone who has not signed up for this lamb and who is not circumcised? The uncircumcised person cannot eat the Passover, and he is not even part of the Seder! He is so far removed from it that slaughtering it for him does not invalidate the sacrifice. Right? - Wrong, says Rav Chisda. Since he could circumcise himself, it is as if he can eat it now.

But wait, Rav Chisda! You yourself did not agree with the principle of since. When we talked about baking at the very end of a Holiday, Rav Chisda did not allow the logic of "since guests might still come and eat it on a Holiday, let's bake!" Here is the answer: Rav Chisda would not allow using since for leniency (like to permit baking). He would allow the reasoning of since for stringency, like to declare the sacrifice invalid.

Art: Woman baking bread by Jean-Francois Millet