He stated this with regard to the prohibition of: "You shall not deviate to the left or the right of that which they tell you" (Deuteronomy 17:11). A prohibition by rabbinic law is overridden by human dignity, but not a prohibition by Torah law. Therefore, Mar bar Rav Ashi would have removed his garment had he known about the tear.
And there are those who say there is a different version of this discussion: It was when they were there, in the place where the corner of Mar bar Rav Ashi's garment tore, that Ravina said to him that it had torn, and Mar bar Rav Ashi said to him in response: What is your opinion? Do you think that I should throw the garment off? But doesn't the Master say: Great is human dignity, as it overrides a prohibition in the Torah? The Gemara raises a difficulty: But Rav bar Shabba interpreted that statement before Rav Kahana: He stated this with regard to the prohibition of: "You shall not deviate," not the prohibition against carrying in the public domain, which applies by Torah law. The Gemara answers that here too, it is not a prohibition by Torah law, as the place where they were walking was not a full-fledged public domain but a karmelit, in which carrying is prohibited by rabbinic law.
MISHNA: The absence of the sky-blue [tekhelet] strings does not prevent fulfillment of the mitzva of ritual fringes with the white strings, and the absence of white strings does not prevent fulfillment of the mitzva with the sky-blue strings. If one has only one, he wears it without the other. Absence of the phylacteries of the arm does not prevent fulfillment of the mitzva of the phylacteries of the head, and absence of the phylacteries of the head does not prevent fulfillment of the mitzva of the phylacteries of the arm. If one has only one, he dons it without the other.
GEMARA: The Gemara suggests: Let us say that the mishna is not in accordance with the opinion of Rabbi Yehuda HaNasi. As it is taught in a baraita: When the verse requires one to place white and sky-blue strings upon the corners of his garments and then states: "That you may look upon it" (Numbers 15:39), it teaches that the lack of either one prevents fulfillment of the mitzva with the other; this is the statement of Rabbi Yehuda HaNasi. But the Rabbis say: The lack of one does not prevent the fulfillment of the mitzva with the other.
The Gemara inquires: What is the reasoning of Rabbi Yehuda HaNasi, i.e., how does he derive his ruling from this verse? The Gemara explains: As it is written: "And they shall put on the fringe of the corner a sky-blue thread" (Numbers 15:38). "The fringe of the corner" is a reference to strings that are of the same type as the corner of the garment. Since garments are usually white, this phrase is referring to white strings. And it is written in this same verse: "A sky-blue thread." And the Merciful One states in the following verse, referring to both types of strings: "And it shall be to you for a fringe that you may look upon it" (Numbers 15:39), in the singular. This teaches that one does not fulfill his obligation until both types are present together.
The Gemara asks: And how do the Rabbis, who hold that the one can fulfill one obligation without the other, understand this verse? The Gemara answers: They hold that the phrase "that you may look upon it" indicates that one fulfills a mitzva with each one individually.
The Gemara concludes its initial suggestion: Shall we say that the mishna, which states that one can fulfill the mitzva with either white or sky-blue strings even in the absence of the other, is not in accordance with the opinion of Rabbi Yehuda HaNasi? The Gemara responds: Rav Yehuda said that Rav said: You may even say that the mishna is in accordance with the opinion of Rabbi Yehuda HaNasi, and the ruling of the mishna is necessary only with regard to granting precedence. The white strings should precede the blue strings, but if the order is reversed, one still fulfills the mitzva.
This is as it is taught in a baraita: It is a mitzva to insert the white strings into the garment before inserting the sky-blue strings, but if one inserted the sky-blue strings before the white strings, he fulfilled his obligation but omitted the mitzva. The Gemara asks: What does the baraita mean by the phrase: Omitted the mitzva?