Rabbi Yonason Sacks
Although the ethics described in Maseches Avos are incontrovertibly a product of the original Sinaic mesorah, no sefer hamitzvos (works of Rishonim which enumerated the 613 mitzvos) reckons these ethics among the 613 canonical mitzvos of the Torah. This salient omission prompts questions pertaining to the mandate for their obligatory observance.
R' Asher Weiss (Minchas Asher, Bereishis 21, Devarim 14) explains that not every mitzvah which a Jew must fulfill is actually written in the Torah or reckoned among the canonical 613. Beyond the written and canonized mitzvos, every Jew is obligated to fulfill the broader meta-halachic category of "ratzon haTorah - the will of the Torah." Although such mitzvos were never actually written in the Torah, the Torah nonetheless makes clear that it desires certain modes of behavior.
For example, the accepted halacha maintains that tza'ar ba'alei chaim - inflicting pain upon animals - is Biblically prohibited (see Bava Metzia 32a). Interestingly, however, the Gemarah itself never cites a source for this prohibition. The Rishonim suggest various possibilities: Rashi (Shabbos 128b, sv. Tza'ar), for example, identifies the mitzvah of prikah - the obligation to assist in the unloading of a burdened animal - as the source for this prohibition, while the Ra'avad (Shitah Mekubetzes ibid. 32b) cites the prohibition of muzzling a plowing animal. R' Weiss explains that neither Rashi nor the Ra'avad would argue that one who inflicts pain upon an animal actually violates these particular commandments; rather, both of these commandments reflect the Torah's disapproval of mistreating animals, thereby rendering tza'ar ba'alei chaim a bona fide Biblical prohibition, despite the absence of a specific source.
Other mitzvos may fall under the category of ratzon haTorah as well. R' Elchanan Wasserman (Kuntrus Divrei Sofrim 22, 23) suggests that all Rabbinic laws fall under the rubric of ratzon haTorah - despite the absence of a specific source, the Torah wills that every Jew should follow the instructions of the Sages. The Chazon Ish (Yoreh Deah 149:8) adds that perhaps the mitzvah of kibbud av v'eim - honoring one's father and mother - may similarly fall under this category. The Gemarah in Maseches Kiddushin cites specific actions which must be performed for this mitzvah: a child is obligated to provide food and drinks for his parents, along with helping his parents dress themselves. The Chazon Ish, based on a comment of the Rashba, explains that although these specific actions fulfill the positive Biblical precept of kibbud av v'eim, nonetheless, the concept of ratzon haTorah dictates that a child do whatever brings pleasure to a parent, even beyond the Gemarah's specific examples.
This concept of ratzon haTorah may also underlie a classic ruling of the Ba'al HaMaor. The Gemarah (Shabbos 134b) teaches that in Talmudic times, all babies who underwent bris milah were bathed in hot water before and after the milah to ensure their safety; failure to do so was believed to pose a significant threat to the baby's life. If a milah was to be performed on Shabbos, the hot water would be boiled before Shabbos for subsequent administration on Shabbos itself (although the act of milah itself overrides the Shabbos, the preparations for a milah do not override the Shabbos). Because the preparatory boiling of the water does not override the Shabbos, in a case where all of the boiled water accidentally spilled from the urn on Shabbos, before the milah could be performed, one would not be permitted to boil new water on Shabbos. In such a scenario, all opinions would agree that the milah must be deferred to Sunday.
The Rishonim debate, however, what the halacha would be if only half of the boiled water spilled out before the milah on Shabbos. In such a situation, may one proceed with the milah? The Ramban (cited by Ran, Shabbos 53a in Rif, s.v. V'heicha) rules that the milah may indeed be performed: the remaining hot water which did not spill will suffice to wash the baby before the milah, and after the milah, the principle of pikuach nefesh - saving a life - will permit the boiling of additional water to wash the baby and save its life. The Ba'al HaMaor, however, disagrees. Although the principle of pikuach nefesh certainly overrides the Shabbos, one is not permitted to intentionally orchestrate a situation in which this permit can be used. For example, if, G-d forbid, an individual suffers an unexpected heart attack on Shabbos, he may certainly violate the Shabbos to save his life; however, to deliberately perform a bris milah without sufficient boiled water, knowing that such an action will inevitably create a situation of pikuach nefesh, is absolutely prohibited.
While the Ba'al HaMaor explicitly prohibits the deliberate invocation of the license of pikuach nefesh on Shabbos, the Achronim debate the nature of this prohibition. R' Shlomo Zalman Orbach (Minchas Shlomo 7:2) reasons that the prohibition is merely Rabbinic in nature: no matter a person's intentions, the Torah itself will always permit a person to violate Shabbos in order to save a life. It was the Sages, however, who felt that such deliberate orchestration was improper. R' Asher Weiss, however, argues that perhaps the Ba'al HaMaor's prohibition constitutes a violation of ratzon haTorah: just as the Torah wills that a person fulfill all of its applicable commandments, so too the Torah wills that a person not intentionally create situations which will exempt himself from its commandments. Accordingly, the Ba'al HaMaor's prohibition could indeed be Biblical in origin, despite the absence of an explicit Scriptural source.
In light of the aforementioned examples, perhaps one could similarly suggest that the source for the ethics prescribed in Maseches Avos is the concept of ratzon haTorah. Although the Torah never states these ethics in a particular chapter or verse, the consistent emphasis upon proper conduct and refinement of character through fulfillment of the mitzvos reveals the Torah's ultimate desire that a person uphold oneself in an ethical fashion.