The Torah, in parshas Bereishis, tells us that God created the world in six days, and rested on the seventh day. Rashi writes that in actuality everything was created on the first day, but the different parts of creation were set in their place during the six days. In regard to the second day of creation, the Torah tells us, "God said, ' Let there be a 'rakiya' (translated as 'firmament') between the waters, and let it separate between water and water.' God made the rakiya, and he separated between the waters which were beneath the rakiya and the waters which were above the rakiya, and it was so. " (Bereishis, 1, 6-7). Rashi explains that even though the heavens were created on the first day, they were still moist until the second day, when they were solidified through God's command for the formation of the rakiya. Rashi also points out, from a midrash, that the Torah does not say, at the end of the second day, that God said it was good. The reason for this, he explains, is that the work of the creation of the water was not completed until the third day, when the water cleared a way for the dry land to appear. Since the work was not yet complete, God could not call it 'good.'
Interestingly, there is another midrash, not cited by Rashi, that gives a different reason for God's not saying 'it was good' after the creation of the rakiya. Since the rakiya causes a separation, the day that the rakiya was fixed in the heavens cannot be called good, since division, or in a broader sense, controversy, is not good. Rashi, however, does not cite this midrash. I believe that Rashi is being consistent with his explanation that everything was created on the first day, but was fixed in its respective place on subsequent days. Following this approach, the fixing of the rakiya in the waters, the act of separation, had an intrinsic value, even though the work of the water was not yet complete. In fact, the Zohar, cited by Rabbi Menachem Kasher in his Torah Shleimah to our parsha, notes the distinction that we find in Pirkei Avos between good and bad controversies, and says that the rakiya was, literally a division for the sake of heaven, and, therefore, endured. The proof that it was enduring, says the Zohar, is in the verse, "And God called the rakiya 'Heavens' (1, 8).
To expand on the idea articulated by the Zohar, division is a very important aspect of Torah learning. Rav Yosef Dov Soloveitchek, zt'l. for example, pointed out that his grandfather, Rabbi Chaim of Brisk, was a great admirer of the rabbinic work Minchas Chinuch, because of its tendency to divide broader Torah concepts into fine distinctions, rendering, in most cases, two distinct ways of looking at a particular. The Talmud tells us that, for this reason, the blessing of havdalah, or separation, that we include in Shemoneh Esreh of mariv after the conclusion of Shabbos and Yom Tov is inserted in the blessing for da'as, or understanding, because the two processes are intrinsically connected. The separation of the waters can thus be seen as symbolic of the distinctions made within the process of Torah learning, since Torah is compared to water, as the Talmud tells us, based on Biblical verses. In fact, the Tikunei Zohar, as cited by the Yalkut Reuveini, says that the separation made by the rakiya represents the separations made in the Torah between that which is permitted and that which is forbidden. This kind of separation is, obviously, for the sake of heaven, so that the separation per se is worthy of being called good. Therefore, it is only because the work of the waters, the completion of the separation process, did not occur until the third day that God did not mention the word 'good' in reference to the second day, but not because separation, division, is, in and of itself, not good.
Rav Avraham Yitzchak HaKohein Kook, in his commentary to the siddur, Olat Reiah, explains, in a similar way, the Talmudic statement that Torah scholars increase peace in the world. On its face, this statement seems problematic, because we find divisions and debates on virtually every page of the Talmud. However, says Rav Kook, this question can only arise from a false view of what peace is. Peace, he says, is not the obliteration of distinctions. Rather, it is the process of making divisions, bringing out the different opinions, and determining the proper place and role of each one. A division of opinions among Torah scholars, then, is a true division for the sake of heaven, and brings true peace along with it.