#802: “But it’s only a D’Rabbanan, anyway!”
There is a principle in halachah: sfaika d'Rabbanan l'kula (when there is doubt regarding [observance of a] Rabbinic law, it is ruled leniently), as opposed to sfaika d’Oraysa l’chumrah (when there is doubt regarding [observance of a] Biblical law it is ruled stringently). There are many exceptions, however, to applying leniency to a d’Rabbanan, some of which relate to the nature of the safek (doubt) and others to the circumstances of the halachah. Below are some of the exceptions:
From the Source: Many of the institutions of Chazal are based on pesukim (verses) from the Neviim (Prophets), called divrei kabbalah (lit., words [of Hashem] that were received). These laws are in some ways akin to those laws that are min haTorah, and therefore, according to many poskim, do not fall under this leniency.
It’s Biblically indicated: At times a d'Rabbanan is established on account of a possible chashash d'Oraisa (prospect of [transgressing] a Biblical law), or the situation originally involved a d'Oraisa but later developed into a d'Rabbanan such as issurim (prohibited substances) that shift from one to the other because a certain amount of time has passed or circumstances have changed. In these cases, the kula is suspended.
Shabbos rite of way: There is a principle of chomer Shabbos (the severity of Shabbos)—when a d’Rabbanan is instituted to preserve the laws and sanctity of the holy day, this often precludes leniency. For example: In most circumstances, it is an issur d’Rabbanan for a Jew to benefit from the melachah ([prohibited] work on Shabbos) performed by a non-Jew. However, if electricity is engaged by a non-Jew for a mixed gathering that is primarily non-Jewish, the Jew may take advantage. What if we are not sure how the crowd divides up? We are stringent in this regard and refrain from benefitting even though the nature of the transgression is d’Rabbanan because of chomer Shabbos.
Not before, but after: "Ein oisin safek d'Rabbanan l'chatchila" (we don't engage in something that's a doubt in Rabbinic law at the outset). Only if we find ourselves in an untenable situation b’dieved (post-facto), can we be lenient. The concept of l’chatchila includes promoting the fulfillment of a Rabbinic mitzvah in doubt if it can easily be performed, and not circumvent it with the “excuse” of sfaika d'Rabbanan l'kula.
Need to know: If the issue of doubt is chesron yedia (lack of knowledge), we do not automatically rely on sfaika d'Rabbanan l'kula. A person is required to investigate and attempt to clear up the doubt if at all possible—unless, of course, the whole world is not privy to the information.
It’s not an “out”: Sometimes the doubt is formed by two (or more) opinions and they are mutually exclusive. If the safek is resolved by ruling leniently on all the possibilities—so that the mitzvah d’Rabbanan would thereby not be performed—the principle is not applied. This situation is found in relation to the requirement for haseivah (reclining) at the Pesach Seder when drinking the four—Rabbinically required—kosos (cups [of wine]). There is a dispute whether reclining is essential for the first two or for the last two cups. If we apply the kula, we would not recline for drinking at all! Therefore, in practice, we recline for all of them.
A Double Doubt: In the event of a sfeik sfaika l’chumrah (a doubt in two areas of stringency)—if there is halachic premise to act stringently on both matters in question, the stringency is bolstered and we do not act l’kula. (For an overview of sfeik sfaika l’kula, see Halachah #688.)
Advocating patience: There is no leniency on a davar sheyesh lo matirin (matter that will [eventually] become permissible). An example of this is an egg that may have been laid on Yom Tov. A newly laid egg is assur d’Rabbanan on Yom Tov (because of muktzah or other reasons). When there is an unresolvable safek whether it was laid on Yom Tov, we are stringent and wait the allotted time for it to become permissible.