As we have learned, a firstborn kosher animal must be given to a kohen and brought as a sacrifice in the Temple. If the animal develops a mum – a blemish that precludes it from being sacrificed – the animal becomes the property of the kohen who can slaughter it and eat it. Given the financial interest that a kohen has in determining whether or not the animal has a mum, the kohen is required to show the animal to an expert who will determine the animal’s status.
What if the kohen did not show the animal to an expert?
If one slaughtered the firstborn and then showed its blemish to an expert, Rabbi Yehuda permits, whereas Rabbi Meir says since it was not slaughtered by the instructions of the expert, it is forbidden.
In the Gemara, Rabba bar bar Ḥana teaches that in cases where the appearance of the blemish may change after death both Rabbi Meir and Rabbi Yehuda agree that the animal cannot be eaten. Their disagreement is limited to cases in the body of the animal itself, which do not change (e.g., if there is a cut in the animal’s ear). Rabbi Meir rules that we prohibit blemishes in the body of the animal on account of those blemishes that change; Rabbi Yehuda does not believe that one type of blemish should affect the status of the other type of blemish.
The example of a blemish that changes after death is dukin she-ba-ayin – a type of eye condition.
Dukin she-ba-ayin is a condition discussed only in the Babylonian Talmud, and its precise definition is not clear. Rashi appears to connect it with the condition that the Torah calls duk in the list of bodily blemishes (see 21:20), which he understands (based on Yeshayahu 40:22) to mean a shade or veil, i.e. a cataract on the eye. Another possible explanation presented by Rashi is that it is a blemish on the eyelid.
Others suggest that dukin she-ba-ayin is a general term for minor eye defects. For example, spots on the cornea are a fairly common eye problem stemming from scarring. This explanation would help us understand the Gemara, since shortly after death the cornea becomes cloudy, so it would be impossible to tell whether the cornea was scarred during the animal’s lifetime.