1) Tosfos ib Sukah (31a, DH v'RI) writes that Semuchin can be expounded from the Torah. It seems that Tosfos is saying that a law derived through Semuchin is considered as written explicilty in the Torah, and it is not necessary to apply here the 13 Midos.

For example, Semuchin tells us that the Torah states explicitly that Kil'ayim is permitted in Tzitzis.

2) Tosfos there in Sukah writes that a Hekesh is where two things are linked to each other in the same verse. Semuchin are two verses which are next to each other and the fact that they are next to each other tells us that we are to compare the two things in the adjacent verses.

3) The Maharitz Chiyus (Sukah 31a) writes that a Hekesh is not one of the 13 Midos. It is not counted among the 13 because a Hekesh is considered a though it is stated explicitly in the verse. He cites Rashi in Sanhedrin (73a, DH Hekesha) who writes that a Hekesh is considered as being written explicitly in the Torah.

4) Even though the above principle refers to a Hekesh, whereas Semuchin is slightly weaker since is not actually in the very same verse, nevertheless Tosfos writes that Semuchin are "Nitnu li'Daresh Min ha'Torah," from which we may learn that the same way that a Hekesh is considered explicit in the Torah, so, too, Semuchin is considered as explicit in the Torah, so it is not one of the 13 Midos.

5) The opinion of the Yavin Shemu'a:

a) I found, bs'd, that the Sefer Yavin Shemu'a (Klal 119) writes that Semuchin in fact does fit into the 13 Midos. Yavin Shemu'a, by Rav Shlomo Algazi zt'l, is a Sefer about the rules of Chazal, printed on the page of Sefer Halichos Olam, which is also a Sefer about the rules of Chazal. (Klal 119 of the Yavin Shemu'a appears in Halichos Olam, Sha'ar 4, paragraph 24, underneath the Halichos Olam.)

b) The Yavin Shemu'a writes that Tosfos here (4a, DH v'Ki) holds that we can assume that all the Tana'im expound Semuchin unless we know specifically otherwise. This is because Tosfos asks, "Should we say that Rebbi Akiva does not hold of the Rabanan who expound Semuchin?!" It seems that it is obvious to Tosfos that Rebbi Akiva expounds Semuchin.

c) The Yavin Shemu'a goes further and writes that we can now also assume that Rebbi Yishmael expounds Semuchin, since we never heard specifically otherwise. He asks that if Rebbi Yishmael expounds Semuchin, then why does he not include Semuchin in the 13 Midos? The Yavin Shemu'a answers that Rebbi Yishmael indeed includes Semuchin in his list of Midos sheha'Torah Nidreshes ba'Hen: it is included in his Midah 12, "Davar ha'Lamed me'Inyano and Davar ha'Lamed mi'Sofo." He writes that the Limud of the Gemara here about Anusah is certainly Davar ha'Lamed me'Inyano because the Semichus tells us what the verse means. Our Gemara wants to know the meaning of the verse, "He shall not uncover the robe of his father." We learn the meaning from the verse stated above in the Torah, which refers to rape. This is a Davar ha'Lamed me'Inyano; it is derived from the context, namely, the previous verse.

d) The Yavin Shemu'a writes that "even the Semuchin of Sha'atnez to Tzitzis is also Davar ha'Lamed me'Inyano, even though we know the simple meaning of the verse without the Semuchin. This is because the Davar ha'Lamed me'Inyano tells us, 'You shall make strings' -- from what do you make strings? From everything, even from Sha'atnez."

6) According to Rabeinu Shimshon of Kinun, Rebbi Yishmael agrees with Rebbi Yehudah:

a) The Sefer Kerisus, written by one of the Rishonim, Rabeinu Shimon of Kinun (who lived in France shortly after the Ba'alei ha'Tosfos) in part 1, Batei Midos, Bayis 2, Cheder 14, writes that possibly one can say that the reason why Rebbi Yishmael does not mention Semuchin as one of the 13 Midos is because he agrees with Rebbi Yehudah who does not expound Semuchi. Even though Rebbi Yehudah does expound Semuchin in the Book of Devarim, Rebbi Yishmael still did not mention Semuchin because Rebbi Yishmael only enumerates Midos by which the entire Torah is expounded, not merely part of the Torah.

b) So far I have discussed only why Rebbi Yishmael does not mention Semuchin, and I have not discussed why Rebbi Eliezer the son of Rebbi Yosi ha'Glili, who lists 32 Midos, does not mention Semuchin. However, it seems that we cannot answer in the same way that the Sefer Kerisus says about Rebbi Yishmael, because the Gemara in Pesachim 28b states that Rebbi Yosi ha'Glili learns that in the year that they came out of Egypt the prohibition of Chametz applied for only one day, from the fact that the verse (Shemos 13:3) states, "Chametz may not be eaten," and immediately afterward (13:4) states "today you are going out." We see that Rebbi Yosi ha'Glili does learn Semuchin so it would be difficult to say that his son Rebbi Eliezer did not learn Semuchin. So we will have to rely on one of the answers that we wrote above to explain why Rebbi Eliezer does not list Semuchin.

7) The idea that Semuchin is included in the Midah of Gezeirah Shavah:

a) I made a rather surprising find in the Sefer Midos Aharon. This is a Sefer on the Beraisa of Rebbi Yishmael, written by Rav Aharon Iben Chaim (1555-1632) of Morocco. At the end of chapter 3 (page 82), he cites Rabeinu Shimshon of Kinun who says that Rebbi Yishmael did not mention Semuchin because he holds like Rebbi Yehudah, but the Midos Aharon disagrees with this for a few reasons:

(1) The fact that a Midah is not used in the entire Torah does not stop it from being called a Midah, so even though it only applies in Sefer Dvarim according to Rebbi Yehudah, it is still called a Midah.

(2) Rebbi Yishmael said, "13 Midos with which the Torah is expounded," and he did not say, "... with which all of the Torah is expounded."

(3) Rebbi Yehudah does expound Semuchin where it is "Muchach" and "Mufneh." If it is clear that one can learn from the adjacent verses, and if the verses are free to be expounded, then Rebbi Yehudah does expound Semuchin (see Tosfos to Sukah 31a, DH v'RI).

These arguments all show that Rebbi YehudaH also holds that SemuchiN is a Midah, at least in suitable circumstances. Why, then, does Rabeinu Shimshon of Kinun say that Rebbi Yishmael does not mention it, even if we say that he holds like Rebbi Yehudah?

b) The Midos Aharon answers that Semuchin is included in the Midah of Gezeirah Shavah. A Gezeirah Shavah compares and equates two subjects. Hekesh and Semuchin also compare and equate two subjects, so Hekesh and Semuchin are included in Gezeirah Shavah, which is why Rebbi Yishmael did not need to list them among the 13 Midos. (This approach does not fit with Tosfos to Bava Kama 83b, DH ha'Chovel, who understands that Gezeirah Shavah and Semuchin are not the same. -DB)

Kol Tuv,

Dovid Bloom