R. Huna said: Tefillin may be written upon the skin of a clean bird. R. Joseph demurred: What does he inform us? That it has a skin! [But] we have [already] learnt it: He who wounds it is culpable? Said Abaye to him, He informs us much. For if we [deduced] from our Mishnah, I might object, Since it is perforated all over, it may not [be thus used]; hence he informs us as they say in the West [Palestine]: Any hole over which the ink can pass is not a hole.

R. Zera objected: [And he shall rend it] by the wings thereof: this is to teach that the skin is fit. Now if you think that it is [a separate] skin, how can Scripture include it? — Said Abaye to him, it is [indeed a separate] skin, but the Divine Law includes it. Others state, R. Zera said: We too learnt thus: 'By the wings thereof'; — this is to include the skin. Now, if you say that it is [a separate] skin, it is well: hence a verse is required for including it. But if you say that it is not skin, why is a verse required for including it? Said Abaye to him, in truth I may tell you that it is not [a separate] skin, yet it is necessary. I might argue, Since it is covered with splits [holes], it is repulsive. [Hence] we are informed [otherwise].

Mar son of Rabina asked R. Nahman b. Isaac: May tefillin be written upon the skin of a clean fish? If Elijah will come and declare, he replied. What does 'if Elijah will come and declare' mean. Shall we say, whether it has a [separate] skin or not, — but we see that it has a skin? Moreover we learnt: The bones of a fish and its skin afford protection in the tent wherein is a corpse! Rather [he meant]: If Elijah comes and tells [us] whether its foul smell evaporates or not.

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Our Rabbis taught: Tefillin can be written upon the skin of clean animals and upon the skin of clean beasts, and upon the skin of their nebeloth or terefoth, and they are tied round with their hair, and sewn with their tendons. And it is a halachah from Moses at Sinai that tefillin are tied round with their hair and sewn with their tendons. But we may not write [them] upon the skin of unclean animals or upon the skin of unclean beasts, and the skin of their nebeloth and terefoth need not be stated; nor may they be tied round with their hair or sewn with their tendons. And this question a certain Boethusian asked R. Joshua the grits dealer: How do we know that tefillin may not be written upon the skin of an unclean animal? Because it is written, 'that the law of thy Lord may be in thy mouth' [implying] of that which is permitted in thy mouth. If so, they should not be written on the skin of nebeloth and terefoth? Said he to him, I will give you a comparison. What does this resemble? Two men who were condemned to death by the State, one being executed by the king and the other by the executioner. Who stands higher? Surely he who was slain by the king! If so, let them be eaten? The Torah saith, Ye shall not eat any nebelah, he retorted, yet you say, let them be eaten! Well spoken! admitted he.