Halacha != Aggada (sources)

1 - I know it is possible that you will seek and find words of individuals from among the scholars of truth, our Rabbis, peace be upon them, in the Talmud and in the Mishnah and in the Midrashot, that indicate that at the time of a person’s birth the stars caused such and such. This should not be difficult in your eyes; for it is improper that we should abandon operative Halakhah and go about [seeking] objections and resolutions. And likewise, it is inappropriate for a person to abandon words of sense, whose proofs have already been verified, and empty one’s hands of them, and rely upon the words of a [solitary] individual from among the Sages, peace be upon them, when it is possible that something was overlooked by him at that time or that those words contain an allusion or [that] he said them at the moment [based upon] an incident that took place. --Iggerot ha-Rambam, II, 488

2 - ...if the words of the ancient [Sages] contradict the intellect, we are not obliged to accept them. -- R. Yehudah ben Bilam of Toledo, commentary on Sam. I,ch. 28 (cited in OHG, “Teshuvot al Hagigah 4b) quoting Rabbi Shemu’el ben Hofni

3 - And it is impossible for us to believe in the veracity of a matter for whose negation there are corroborations, only because some of the ancient [Sages] said it. Indeed, it is necessary that we contemplate the matter with our intellect. If a proof may be found for its veracity, we shall accept it. If there comes corroboration for its possibility, we shall believe in it as something possible. And if it is found to be impossible, we shall regard it as impossible. --Perush ha- Torah le-Rabbi Shemu’el ben Hִofni

4 - Aggadah is every explanation that comes in the Talmud regarding any matter that is not a mizvah. This is Aggadah; and you should learn from it only what arises in the mind.… What [the Sages] interpreted in [Scriptural] verses is [for] each one according to what occurred to him and what he saw in his mind. And according to what arises in the mind from these interpretations, one learns it; and one does not rely upon the rest. --R. Shemu’el ha-Naggid’s Mevo ha-Talmud

5 - One should not rely upon Aggadah and Midrash, even though they are written in the Talmud, if they are unattuned or erroneous. For our principle is: One does not rely upon the Aggadah. Rather, what is fixed in the Talmud, [in] which we find [the means] to remove its error and reinforce it — we should do so; for, if it had no basis, it would not have been fixed in the Talmud. And what we do not find a way to clear of its error — becomes like matters that [do] not [accord with the] Halakhah. [With] what is not fixed in the Talmud, we need not [do even] this much. Rather, one ponders it; if it is correct and becoming, one expounds it and teaches it; and, if not, we pay it no attention. --Sefer ha-Eshkol

6 - Words of Aggadah are not like a tradition; rather, everyone expounds what arises in his heart — like ‘it is possible’ and ‘one may say’ — not a decisive statement. Therefore, one does not rely upon them. --Chai Gaon

7 - ...words of Aggadah, to draw the heart of man ... if you desire — believe them; and if you do not desire — do not believe them, for no law is determined based upon them. --R. Yehִ i’el ben Yosef of Paris, Sefer ha-Vikuahִ

8 - We have a third book called Midrash, meaning sermons. It is just as if the bishop would rise and deliver a sermon, and one of the listeners whom the sermon pleased recorded it. With regard to this book [of sermons], if one believes in it, it is well and good; if one does not believe in it, he will not be harmed [spiritually] -Ramban, Disputation at Barcelona

9 - Consider also the more extreme attitude toward Aggadah ascribed by the Yerushalmi to the talmudic sage R. Ze‘ira, who branded aggadic works “books of divination,” “from which we understand nothing” definitive — urging his preeminent disciple, R. Yirmeyah, to avoid them altogether in favor of halakhic study (Yerushalmi Ma‘aserot 3:4)

10 - It is possible that some of [the stories in the Talmud and Midrash] were stated only in the manner of rhetorical invention [Heb. hamzִ a’at melizah], for some ethical or educational goal. And even if one says that the [apparently historical] stories of Avraham’s life with Terahִ and Nimrod in Ur Kasdim were a rhetorical invention — [we] shouldnot reject [that position] with two hands -- R. Samson Rephael Hirsch

Thanks to Chardal for providing me with an essay written by Rabbi Chaim Eisen which argues conclusivly that midrashim are not canonical. The sources cited above (along with many others) are all more or less verbatim from Rabbi Eisen's endnotes.

(PS: Dear Yus and Ed. Eat it XXOO DovBear)