The requirement of inspecting the knife of a shohet is not mentioned in any Mishnah or Baraita. On the contrary we learn in a Mishanh (Hullin 15b), one may perform shehitah with any knife other than a harvest sickle or a saw. But if an uninspected knife is considered defective, why did the Mishnah exclude only a harvest sickle and a saw, which are specialized implements with serated edges across their entire length rendering them defective for shehitah, when it would be forbidden to use any knife with even a single rough edge to perform shehitah? Nor could the comment in the Gemara that, in teaching us that it is forbidden to use any knife with defects similar to a sickle or a saw to perform shehitah, have been referring to any knife shaped like a saw or a scythe, because the Mishnah contrasts the harvest sickle with a hand sickle which it is permitted to use in performing shehitah. But the intent of the Mishnah must have been to teach us that any defect in a knife that is similar to the defects of a harvest sickle or saw (i.e., that is likely to tear not cut) is disqualified for use in performing shehitah.

It is only from a dictum of R. Hisda, a late amora, that we first learn about a requirement to inspect a shohet’s knife. And this requirement is communicated in a very remarkable way; R. Hisda asks from what Biblical source do we know that the knife of a shohet must be inspected? It says “and you shall perform shehitah with this” (ושחטתם בזה), a verse (1 Samuel 14:34) recording the command of King Saul.

Now the question that ought to have been asked about R. Hisda’s teaching is that his proof text is from the Prophets not from the Five books of Moses, and Biblical obligations are inferred only from the Five Books written by Moses, not from the Prophets. Moreover, this requirement is clearly supplementary to the Biblical mandate, the Torah writing only “ושחט” or “וזבחת”, but not “בזה”. It was only Saul who added “בזה” (with this), the Torah specifying no requirement to inspect a shohet’s knife either before or after shehitah is performed. So how could R. Hisda have asserted that it is Biblically required to inspect the knife?

But rather than raise this question, the Gemara poses the opposite question: that it is obvious that if an animal’s siman is punctured instead of cut, the animal is rendered a tereiphah. The implication of the question is that no source is required to teach that the shohet’s knife must be inspected, because every shohet will inspect his knife as a matter of course, in order to be certain that a defect in the knife being used to perform shehitah will not lead to a defective shehitah rendering the animal a tereiphah, making it inedible by Jews. The answer offered by the Gemara to this question is that the requirement for inspection of the knife must be fulfilled by a scholar (חכם), not by the shohet or a layman. The discussion concludes that inspection of a knife requires the blade to be felt by flesh and nail on its three sides, and any shehitah performed with an uninspected knife is invalid. But it is astonishing that the halakhic conclusion of the sugya that a knife must be inspected by feeling the blade of the knife with flesh and nail on its three sides when the law stated in the Mishnah is that one may perform shehitah with any knife other than a sickle or a saw, because those implements would tear rather than cut the simanim.

As will be shown, the Gemara and the Mishnah can be explained only according to the Rambam. The explanations of both Rashi and the BeHag are problematic, because, according to Rashi, the defect in shehitah known as iqur (tearing) refers to a shehitah performed with a blemished knife, while the BeHag (Ba’al Halakhot Gedolot) does not consider iqur caused by using a blemished knife when performing shehitah to be one of the defective forms of shehitah (See Tosaphot, Hullin 9a (ד”ה כולהו)) because it is not even in the category of shehitah, but rather a form of choking (חניקה). Neither the explanation of Rashi nor that of the BeHag can be comprehended. Why, according to the BeHag, should performing shehitah with a blemished knife be excluded from the category of shehitah any more than cutting the simanim while applying downward pressure? And why, according to Rashi, did the Mishnah disqualify a sickle and a saw from being used to perform shehitah without any mention of a blemish in an ordinary knife that could also cause a tear in the simanim?

Thus, the Hatam Sofer, in his Responsa (Yoreh Dei’ah siman 15), asks: “Was it a blemished knife that the laws of shehitah transmitted to Moshe prohibited? Rather, it was tearing the simanim, pausing while cutting the simanim, applying pressure to the simanim, cutting the simanim at a slant and cutting the simanim outside the required area that were transmitted to Moshe at Sinai.” Inasmuch as there is neither Scriptural a basis nor a halakhic tradition concerning a shehitah performed with a blemished knife, but only the Biblical requirement to cut, but not to tear or to apply pressure, it is inconceivable that failure to inspect the knife with which a shehitah is performed would cause an otherwise valid shehitah to be invalidated, because as long as the shohet did not feel that there had been a tear while performing the shehitah, only the continuous back and forth motion of the knife, the shehitah is presumptively valid. Since “cutting” (שחיטה) is different from “tearing” (קריעה), and the difference can be felt by shohet or detected upon inspection of the animal after shehitah, the assumption that a blemish in the knife is sufficient to invalidate a shehitah is problematic. Moreover, it is astonishing that the Gemara questions that it is obvious that there is a requirement to inspect the knife before performing shehitah, because any puncture in a siman renders the animal a tereiphah. But why did the Gemara ask its question in terms of a puncture of the siman when the issue under discussion was to inspect the knife of the shohet for a blemish; the question of the Gemara should therefore have been it is obvious that there is a requirement to inspect the knife, because if there is a blemish in the knife, performing shehitah with that knife will cause the animal to be rendered a tereiphah. In his glosses on the Gemara, the Maharatz Chayes makes the following comment on this passage (“פשיטא דכי נקב טרפה”)

See the Shakh (Yoreh Dei’ah 19:4) who disputes the Drisha’s explanation that inspecting the knife before performing shehitah is derived from the verse “ושחטתם בזה”, . The Shakh rejects that explanation based on this passage, wherein the Gemara asserts that the requirement to inspect the knife before shehitah may be inferred even without a verse. It is only the requirement that the knife be presented for inspection to a Torah scholar that is inferred from the verse “ושחטתם בזה”. But the Shakh did not recall the words of the BeHag at the beginning of hilkhot shehitah, quoting R. Hisda, “From where to do we know that there is a Biblical obligation for a knife to be inspected? For it is written “ושחטתם בזה”, which implies either that he had a different text of the Gemara or, more likely, that he understood the Gemara to be asking that, inasmuch as puncturing a siman renders the animal a tereiphah, the requirement to inspect the knife before shehitah is obvious.

What the Mahartz Chayes meant is that because the Gemara asks “פשיטא דכי נקב טרפה” rather than  “פשיטא דכי פגום טרפה” the question of the Gemara must have been: obviously if the shohet, while performing the shehitah, perceives that he has punctured or torn a siman, or if, upon inspection, he sees that the siman is torn, the animal is rendered a tereiphah, so it follows that, to ensure that his shehitah will not be invalidated, the shohet must inspect his knife before performing shehitah. But why must this requirement be deduced from Scripture? The Gemara tries to answer this question by suggesting that we are seeking a Scriptural source only for the obligation to present the knife to a Torah scholar. But this answer also requires explanation, as does the subsequent question: did not R. Yohanan say that the requirement to present the shohet’s knife for inspection to a Torah scholar was not essential for shehitah but is required merely as a courtesy to the Torah scholar (“מפני כבודו של חכם”).

But the response to the question that the requirement is merely rabbinic, the proof text being merely an asmakhta, not a legitimate inference, is difficult to understand, because if showing the knife to a Torah scholar is only a courtesy, there isn’t even a rabbinic obligation to present the knife to a Torah scholar. If so, why would R. Hisda have asked “from where do we know that there is a Biblical obligation for a shohet’s knife to be inspected?” If the knife is presented to the Torah scholar only as a courtesy, his question seeking further explanation, let alone a Scriptural source, for the practice is out of place.

However, the position of the Rambam is that the requirement to present a knife with which a shehitah will be performed to a Torah scholar was instituted because of a halakhic concern about the shehitah, not as a mere courtesy to a Torah scholar, because he states (Hilkhot Shehitah 1:26) that any shohet who did not have his knife inspected by a Torah scholar before performing shehitah is banned as a shohet, even if his shehitah in that instance was not defective, because of a concern that he may again rely on himself to inspect the knife before performing shehitah with a knife that actually is blemished. And much ink has been spilled in futile efforts to reconcile the Rambam’s opinion with the Gemara, but in truth the reason that the knife must be presented to a Torah scholar for inspection is that even a minor blemish in the knife that is too small to cause a tear in the siman also invalidates the shehitah. The invalidation is surely just rabbinic, based on the verse “ושחטתם בזה”, which is interpreted to mean that it is not enough to perform shehitah without tearing the siman, but it is necessary to perform the shehitah בזה “with this,” meaning with a knife like this one, a knife totally unblemished, even without a blemish too small to cause a tear in the siman, because even the slightest blemish in the knife causes the shehitah to be rabbinically invalid.

But to find such a minimal blemish requires a high degree of sensitivity and concentration, so that only a Torah scholar, not just an ordinary shohet, can be relied upon to perform or supervise the inspection. The rabbinic requirement for inspection of the knife by a Torah scholar is thus a real halakhic necessary not a mere courtesy to the Torah scholar. A blemish in a knife is rabbinically disqualilfying even if it is too insignificant to cause the shehitah to be defective. And to detect such a blemish requires a Torah scholar who has the concentration and sensitivity and power of concentration to detect such a minimal defect. The assumption that the obligation to present the knife to a Torah scholar was only a courtesy was only the opinion of R. Yochanan, of the first generation of Amoraim, when the rabbinic obligation to inspect the knife of a shohet had not yet been generally accepted. However, by the time of R. Hisda, a late Amora, the obligation had been generally accepted and the verse “ושחטתם בזה” was relied on as a source for the Rabbinic obligation for the shohet’s knife to be free from even a minimal blemish..