If Rashi on Tanach is a purely pshat oriented commentary, as professed several times, why do many of his comments originate from midrashim that most other "pshat" meforshim dismiss as "derech drash"? [For examples, see Ramban (Gen. 21:17), Ibn Ezra (Gen. 32:9), Radak (Gen. 21:7), Ralbag (Josh. 24:32), and Abarbanel (Gen. 23; first question).]
Did Rashi have a different definition of pshat and drash?
The original basis for saying that Rashi is always saying peshat is his statement on Bereishit 3:8: ואני לא באתי אלא לפשוטו של מקרא ולאגדה המיישבת דברי המקרא דבר דבור על אופניו
But note the end of the statement, ולאגדה המיישבת דברי המקרא דבר דבור על אופניו. Thus, some or much of what Rashi says is admittedly aggada, but which works well with the peshat level and resolves something. Indeed, the quote from the Lubavitcher Rebbe from Likutei Sichot even lops off the end of the quote. I personally feel that people, including many great meforshim of Rashi, have overapplied this statement. (Other Rashi scholars agree that it should not be overapplied.) I elaborate a bit on this point here.
Besides this, "peshat" means different things. For example, it could mean an explanation on the peshat level of interpretation, or it could mean that only correct explanation on the peshat level. And so, one should point out that many other meforshim (Ibn Ezra, Ramban, Seforno, etc.) argued on Rashi.
It is true that there are different kinds of peshat. For example, we have Rashbam's comment (to Gen. 37:2) that Rashi himself, who aimed at peshat (נתן לב לפרש פשוטו של מקרא) agreed that new peshat-based interpretations are needed (והודה לי שאילו היה לו פנאי היה צריך לעשות פירושים אחרים לפי הפשטות המתחדשים בכל יום).
That said, it is worth highlighting the approach of the Lubavitcher Rebbe zt"l. He takes the view that every detail in Rashi's commentary - even to the extent of which words he uses as the catchphrase (דיבור המתחיל), the citation of the name of the Tanna or Amora who gave a particular explanation, and the order in which he gives multiple explanations - is significant on a peshat level. Each of these, he says, implicitly solves some problem that could occur to a child studying Chumash (who has not yet studied Mishnah or Gemara, so that any problems that he thinks of must emerge from the text itself). Numerous talks of his over a period of some twenty years (1964 to about 1988) are based on this approach, which is summarized in the introduction to Likkutei Sichos, vol. 5, as follows (translation is mine):
"The central idea of these explanations, as the Rebbe often explained, is based on the rule that Rashi himself stresses about his commentary (to Gen. 3:8 and 24): 'I have come only to give the simple meaning.' This means that Rashi's comments primarily set forth the peshat way of learning and understanding the Torah's verses - so that, in the light of his comments, they become understandable even to a child who is first starting to study Chumash (at age five, as set forth in the Mishnah). This peshat approach of Rashi's - and the fact that every word of his leads to the simple meaning of the verse - is thoroughly illuminated by these explanations..."
In view of the answers given, I would like to approach the answer to this question as did R' Herczeg (who translated for the Artscroll Rashi series) in his book "Patterns in Rashi", in which he takes an approach similar to Josh's answer.
To Rashi, the line between drash and pshat is vague. He viewed pshat as the simple meaning of the pasuk, but only insomuch as it adheres to it grammatically and contextually. Whenever Rashi could not find a pshat that fit with the grammar of the pasuk, he looked to Chazal's collection of aggados. He felt that if an explanation of chazal fits with the grammar and context of the pasuk, it falls into the category of pshat. You might say he felt that it was "pshat enough", even though it might not be the simplest explanation. Many times we will find Rashi bring the popular pshat explanation, then bring a medrash from chazal. This is because he saw some sort of flaw or redundancy in the pasuk that needed to be explained. To Rashi, that is included in pshat.
Besides for grammar, several other things caused Rashi to side with chazal, including juxtaposition of topics, unusually spelled words, and seemingly unnecessary information given by the Torah. Even though these topics are generally considered, even by Rashi, as needing only to be addressed with drash, and according to pshat they may be ignored, still in certain instances Rashi felt that for some specific reason they interfered with pshat on that occasion, and therefore he may, in such instances, use chazal's aggados to clear things up. He considered this to be consistent with the pshat approach.
Other later rishonim disagreed with Rashi. Meforshim like Ramban, Ibn Ezra, and Rashbam viewed pshat and drash as distinct categories, reserving the simplest explanation of the pasuk and that alone for pshat. What about the grammatical inconsistensies in the pesukim? We will have to explain them away continuing to use pshat. There is no need for the more fantastical explanations of chazal. Those are drash; they have another place, a separate place, in the interpretation of the pasuk.
This is what is meant by the famous argument Rashbam records that he had with Rashi. Rashbam felt that even the irregularities in the text can be addressed with pshat, without the need to resort to aggados chazal. He claims that Rashi conceded to him and admitted that if he had had more time, he would have included in his commentary the simpler explanations that were still consistent with the grammar of the pasuk, as they were "more pshat" than chazal's explanations.
For an extensive discussion on this topic, with an abundance of examples, I highly reccomend R' Herczeg's book.
Prof. R' Mordechai Z. Cohen at Yeshiva University is renowned for his explanations of the different styles of the Parshanim. His explanation of Rashi, to which I cannot do justice, can be basically understood in the following phrase often used by R' Meir Goldwicht (not a phrase R' Cohen uses): 'Omek HaPeshat.
As R' Goldwicht uses the phrase, it means, in a nutshell, taking all of the surrounding context into account before analyzing and approaching the Peshat of the text (in his case, Gemara). R' Goldwicht would highlight what a lot of Yeshivoth call "Gemara Be'Iyun" - Gemara, Rashi, Tosafoth, Ramban, Rambam, Rif, Ran, even Shulhan 'Aruch, Rama, Shach and Taz - as being, really, 'Omek HaPeshat ('Iyun being much more in depth analysis than even most very serious Yeshivah students don't get in their first few of years of learning).
Along similar lines, R' Cohen basically says that Rashi's idea of Peshat is to bring background to help the student understand what is going on in the Pasuk.
Again, this is my own interpretation of R' Cohen's presentation of Rashi's approach to Parshanut. I really am not doing him any justice, but I wanted to highlight it anyway, because I think it is rather brilliant.
Yes, the definition of pshat evolved over the generations, from a more view more inclusive of allegorical, non-literal understanding, to a more literalist, less fanciful view (besides the variability among authors of a single generation). See R. David Weiss HaLivni's book on the topic, "Pshat and Derash"
Unkelos is Pshat. Rashi is answering questions that arise on Pshat.
The novelty of Rashi is that he used Midrash to resolve pshat issues when he was able to tie the midrash to something in the text. Thus he uses midrash to explain the pshat on a pshat like level, and does so only when he can find the midrash in the pshat text.
Rashbam, Rashi's grandson claimed to say that if his grandfather would have done it again, he would have done it the way Rashbam did it. Literal pshat. Therefore, although Rashi's goal was indeed pshat, in the end he regretted his decision of not being completely pshat-oriented.
This question confuses a lot of people, and the confusion arises from people thinking that the word p'shat, the first of the four levels of פרדס, means the same as when we say that Rashi is explaining p'shat - an unfortunate but understandable mistake. Because the two usages of this word have two very different meanings.
The first means the literal meaning of the words of the Torah. The second means the simple explanation of the Torah, how we are supposed to understand what the Torah is saying. This is what the פשטנים - Rashi, Ramban, Ibn Ezra, et al. - are coming to do, and this is why we are commanded the read the weekly Torah portion with the Targun Onkelos and/or Rashi, so that we know what the Torah means to say on a basic level.
Most of the time this basic meaning is the p'shat, the literal meaning of the words, but sometimes what Chazal teach us in the midrash or the gemara is what the Torah means, and thus Rashi brings their explanations as the 'p'shat' - the basic teaching of the Torah.
Rashi most often gives the pashat of the chumash but there are times when rashi is not totally satisfied with what he understands the shat of the text to be and in those cases with also offer additional explanations. Often when looked at as a whole there is something to be learned from why Rashi chose the explanations he did and in the order he did so. Much of lekute sichois are teachings from the Rebbe which take a closer look at Rashi.
