Beitzah 6B

השתא מוכן לאדם לא הוי מוכן לכלבים, דתנן: מחתכין את הדלועין לפני הבהמה, ואת הנבלה לפני הכלבים. רבי יהודה אומר: אם לא היתה נבלה מערב שבת ⁠— אסורה, לפי שאינה מן המוכן; מוכן לכלבים הוי מוכן לאדם?!

Now, we know that food prepared, i.e., fit, for human consumption that became spoiled is not automatically considered prepared for dogs, as we learned in a Mishnah (see 2a): One may cut pumpkins before an animal to facilitate their consumption, and likewise one may cut up an animal carcass, even of an animal that died on Shabbat, before dogs. R' Yehuda says: If the animal was not already a carcass, i.e., it was not dead and fit for dogs, prior to Shabbat, it is prohibited, because it is not in the category of items considered prepared for use on Shabbat. This shows that although this animal was fit for human consumption while alive, it does not automatically become prepared for dogs once it dies. If so, can food prepared for dogs be considered prepared for humans?

אמר ליה: אין, מוכן לאדם ⁠— לא הוי מוכן לכלבים, דמאי דחזי ליה לאיניש לא שדי ליה לכלבים. מוכן לכלבים ⁠— הוי מוכן לאדם, דדעתיה דאיניש אכל מידי דחזי ליה.

He said to him: Yes. It is not surprising that something prepared and fit for humans is not considered fit and prepared for dogs, as that which is fit for a person, one does not throw it to dogs, and he has therefore removed that animal from his mind. However, something that is prepared for dogs is also considered fit and prepared for humans, as a person’s mind is on anything fit to be eaten by him. One does not completely remove from his mind even food meant for dogs, if it is kosher and edible. Consequently, one has in mind the possibility that he might eat the calf of a cow that is a tereifa once it is born, since at that point it will be kosher and edible.