There is seemingly no better proof for a Talmudic viewpoint than support from a biblical verse. Expressions such as dik’teev, “it is written”, or shene’emar, “as it says”, appear on almost every Talmudic page and are used to introduce biblical texts in support of a given view. While sages may argue on how to interpret the verse—and thus, often reach different conclusions—it is the verse that gives backing to their positions.
The primacy of the written word is such that the Rambam’s second principle in his enumeration of the 613 mitzvot is that only that which is specifically written in the Torah can be counted as one of the 613 mitzvot. That which is derived from the Torah—as important as it may be, and despite the greater quantity of these derivations—cannot compete with that which is written in the text.
Most of Jewish law is a product of the Oral Law, but the Oral Law derives its authority from its being based upon the written text—a form of “reading between the lines”, if you will. It is for this reason that an oral tradition with no textual basis is authoritative only when it can be traced back to Moshe Rabbeinu, who received it from Sinai. Yet even such a halacha leMoshe miSinai is not to be equated with the written text. A court cannot inflict penalty without a textual warrant and hence, there are no penalties for violating a halacha leMoshe miSinai.
Yet, as definitive as a Biblical verse may be, the power of a logical argument is just as powerful, perhaps even more so.
The opening Mishna of masechet Menachot teaches that if one brings a korban mincha without proper intent, the korban itself is valid and may be offered on the altar and eaten by the kohanim. However, the one bringing the korban has not fulfilled their personal obligation and must bring a second korban lishma with the proper intent. One should not, in the words of Rava, say that “since I slaughtered an olah shelo lishma, I will throw the blood shelo lishma”. Two wrongs do not make a right, and the fact that part of the korban was done incorrectly does not justify doing any other part incorrectly.
As to the source of Rava’s claim, the Gemara explains that, “If you wish, [we can explain this] through a logical argument, and if you wish [we can cite] a verse” (Menachot 2a). This is a startling claim, one that places the force of logic on par with a Biblical verse. But such is the power of logic that an argument based on solid reasoning—absent any proof—is equated with a Biblical verse, made known to us through Divine revelation.
Moreover, we often find the Talmud taking this concept a step further, questioning why we even need a verse to prove a particular point by noting that the point can be deduced via a logical argument and hence, the biblical proof text adds nothing.
What is fascinating is that the logical argument Rava presents can be debated. One could logically posit that once part of the korban was improperly done, the korban loses its status as a korban and there is no point in offering it. One could even argue that it would be improper to do so, as one is not allowed to bring an invalid korban to the altar. And would it not make sense to argue that when the first act of bringing the korban is improperly done, it must be fully disqualified?
It is in masechet Menachot that the Mishna discusses which mitzvot should be done even if they cannot be completely fulfilled, and which mitzvot lose all meaning if part of the mitzvah is missing. One can and should put on the head (or hand) tefillin despite the fact that one does not have tefillin for one’s hand (or head) (Menachot 38a), but there is no point in taking an etrog if the arava is missing (Menachot 27a). Perhaps bringing a korban is like a holding a lulav and etrog, where a defect in one of the species renders them all unfit.
Yet such possibilities did not matter to Rava. His argument can be defended logically and that is reason enough to advance it, with or without a Biblical verse.
The laws of the Torah are not random decrees from a distant Creator but reflect a merciful, kind and compassionate G-d. Its laws are rational, logical and compelling. Yesterday was the 73rd yahrzeit of Rav Avraham Yitzchak Hakohen Kook. I am reminded of a powerful teaching of his (Shemonah Kevatzim 1:75), that we never use the Torah as a basis for acting in a way we intuit is immoral. It defies logic to think that the Torah would lower our natural sense of morality. We must always keep in mind that “the Torah begins with chesed and ends with chesed” (Sotah 14a).
 Dr. Haym Soloveitchik, in his seminal article “Rupture and Reconstruction” (see the article here), argues that post-war Orthodoxy has shifted from a mimetic tradition to a text-based one. Its emphasis on text extends well beyond the Biblical canon such that long-standing traditions have been turned on their heads because they conflict with a rabbinic text, even if that text had been ignored for generations, or even if that text is of most recent vintage.
 Why some mitzvot can be done incompletely and not others is a topic worthy of individual analysis. In the above examples, we might explain that the four species of Sukkot represent various types of Jews from the most to the least righteous, whereas the hand and head tefillin reflect the worship of G-d with our hearts and intellect. One can have a bifurcated relationship with G-d; it is not all or nothing. However, we must ensure the unity of the Jewish people and even sinners must be welcomed—something we declare publicly as we begin Yom Kippur.
 Of course, if a Torah verse—either directly or through an interpretation—specifically mandates a course of action, we must follow it even if our logic would dictate otherwise. However, such cases are few and far between.