The Talmud teaches that it is not permitted to recite shma in an unclean place, and that one who is in the midst of reciting and encounters such a place must stop. One who is does not stop is criticized with the three possible verses, including “Wherefore I gave them also statutes that were not good, and ordinances whereby they should not live” (Yechezkel 20:25); "Woe to them who draw iniquity with cords of vanity" (Yeshayahu 5:18)”, and “For he scorned the word of Hashem” (Bamibdar 15:31). One who does stop is praised with the verse, “and through this matter you shall prolong your days” (Devarim 32:47).

R. Shlomo Kluger (Chokhmat Shlomo, O.C. 85:1) discusses the diffences between the three suggested verses. He asserts that one who does recite shma under these improper circumstances must repeat shma afterward, and cites the Pri Megadim as basing this on the rule of Mitzvah ha-Ba’ah B’Aveirah, a positive commandment performed through a transgression, which is generally invalid.

R. Kluger posits that the verses represent three different perspectives on Mitzvah ha-Ba’ah b’aveirah. The first view is that a mitzvah peformed through a transgression is valid, albeit without reward. Hence, “Wherefore I gave them also statutes that were not good, and ordinances whereby they should not live”; the action is still called a statute and an ordinance, but not onw that is “good”. The second opinion is that a Mitzvah ha-Ba’ah B’aveirah is completely ineffective, and thus just a transgression. Thus, “cords of vanity” represent the deceptive nature of an act that looks like a mitzvah while is actually a transgression. The third opinion asserts that a Mitzvah ha-Ba’ah B’aveirah is indeed invalid, but only when the transgression is biblical. As reciting the shma in an unclean place would be rabbinically prohibited, that in itself would not invalidate the mitzvah; thus, the biblical verse, “For he scorned the word of Hashem” must be brought.

The question of Mitzvah ha-Ba’ah b’aveirah when the prohibition is rabbinical is a question debated by authorities (see S’dei Chemed, ma’arekhet mem, klal 77:7 and Resp. Torat Chesed, O.C., 31:1-2) It would seem that at least three conceptual questions are at issue here:

a) Whether rabbinic laws are mandated by the strength of biblical obligation (Devarim 17:11: “You shall not deviate from the word that they will tell you, right or left”), a topic of dispute between the Rambam and Ramban (Hilkhot Mamrim 1:2 and Sefer HaMitzvot, shoresh 1). Rashi (Pesachim 35b) is of the opinion that a mitzvah performed through a rabbinic transgression is not valid. The Resp.Hitor’rut Teshuvah (III,376) questions whether the disqualification is biblical or rabbinic, but assumes it is biblical, and that this is a reflection of Rashi’s position (Ber. 19b, s.v. kol milei, Yoma 74a, s.v. v’aliba) that rabbinic law is mandated by the strength of biblical verse.

b) Whether a rabbinic innovation that affects biblical law redefines that biblical law, a topic argued by Tosafot and the Ran (Sukkah 3a) among others. R. Yehousha Ehrenberg (Resp. Dvar Yehoshua IV, 1:5 ) assumes this is the dominant issue in this case.

c) Whether rabbinic prohibitions create an inherent negative quality, an issue debated by achronim. It may be, as discussed by R. Moshe Shternbuch (Mo’adim U’Zmanim, II, 109, and III, 261) that a mitzvah can only be disqualified by an accompanying aveirah if that transgression is one that addresses the inherent quality of the act, rather than an external prohibition on the individual (which many achronim assume is true of all rabbinic prohibitions).