Does one fulfill the Biblical precept to study Torah while studying Rabbinic precepts? R. Mordechai of Vilna writes that one does not. Are there disputants? Please answer with exact sources or powerful inferences.
There are two sources I know of indicating that one does not fulfill the mitzvah of Talmud Torah by learning laws which are derabanan:
There is a law that one may not receive payment for teaching Torah, but the Rama (Y.D. 246:5) writes that this is not true regarding teaching of rabbinic laws. This strongly implies that there's no biblical obligation to study these laws.
- The Aruch Hashulchan (there) writes that he finds this ruling difficult, as the Gemara (Megillah 19, Brachos 5a) indicates that even the rabbinic laws were told to Moshe at Sinai
- R. Ahron Leib Shteinman, (Ayeles Hashachar to Nedarim 37a) writes that although this halacha indicates that studying rabbinic laws such as eiruvin or channukah wouldn't fulfill the mitzvah of Talmud Torah, he writes that this is not the case. Perhaps the mitzvah really is fulfilled, but the rule of teaching Torah without pay doesn't apply because Moshe himself never 'taught' these laws.
According to Talmidei Rabbeinu Yonah (Brachos 11b), one only makes a bracha on studying Talmud or the like because it is an explanation of the Written Torah. (Based on this, the Aruch Hashulchan (47:8) is hesitant to make a bracha on studying aggados, because they do not explain any scriptural verses.)
- It would appear, therefore, that studying laws which do not explain scripture (such as the laws of Chanukah) are not subject to birkas haTorah, probably because Rabbeinu Yonah felt that such study cannot be considered talmud Torah d'oraisa
- However, the Eshel Avraham (47:2) writes based on the Rambam that one does make a bracha on studying purely rabbinic laws as well (could this be a dispute between Rambam and Rabbeinu Yonah?)
Besides for R. Shteinman, here are the words of R. Soloveitchik as recorded in שעירוים לזכר אבא מרי ז"ל, pg 253:
האם נאמר למשל כי מכיון שמצות מגילה היא רק מד"ס גם הלכות מגילה אינן מוטבעות בחותמה של תורה שבע"פ דבר תורה ומסכת מגילה מהווה חפצא של תורה רק מדבריהם שמדאורייתא אין בה חלות תורה כלל וכלל... שהלומד אותה אינו מקיים מצות תלמוד תורה מדאורייתא כי הלא כל הדברים שמסכת זו עסוקה בהם אינם אלא רשות דבר תורה? ...נראה פשוט כי התפיסה השניה נכונה וכי כל דיני מצוות דרבנן חפצא של תורה הויין ואין שום הבדל ביניהם לבין הלכות מילה או חמש הלכות שחיטה
Rav Soloveitchik then shows how this is evident from the Rambam (Hil. Mamrim 1) who includes the laws made by the rabbis as התורה אשר יורוך.
Building on this principle, Rabbi Zvi/Hershel Schachter writes in Nefesh Harav (pg. 41) that these laws become an integral part of the Torah itself, and therefore studying them is very much a fulfillment of the Biblical command. He relates this to the rule that כל דתקון רבנן כעין דאורייתא תקון, the rabbis pattern their enactments similar to the ways of the Torah, because it's all one inter-related package. In person, I've heard him take this a step further: there are times when one can understand a biblical law better because we can extrapolate from a detail in a rabbinic law which was patterned after the biblical one (he pointed to Kovetz he'Aros of R. Elchanan Wasserman, no. 66 for examples). We therefore see that even rabbinic enactments can indirectly clarify scriptural verses, perhaps answering the problem posed by the implication from Rabbeinu Yonah above.
Additional evidence that one doesn't fulfill a biblical obligation can be adduced from Rambam's opinion that one is obligated to teach his son specifically the written law (Hil. Talmud Torah 1:7). In general Rambam (e.g. sefer hamitzvos assei 11) parallels the obligations of personal study and teaching ones sons as parts of the commandment "v'shinantem l'vanecha" as noted by Maharam Shik (cited in sefer hamafteach to talmud torah 1:7). This would imply that one's personal biblical obligation to study also includes only the written law.
Maharam Shik himself however assumes that the obligation to teach students, and presumably one's personal obligation does include the oral law, and therefore doesn't understand Rambam's ruling about teaching specifically the written law.