The Torah is unlike the kind of literature that we are used to in our day. The novels, essays and informational materials that we read are composed to transmit a limited and well- defined amount of materials efficiently and easily. In the era of information overload, it is crucial to communicate efficiently and with clarity; contemporary readers can hardly be expected to plumb, mine or unravel, not would they consent to do so.
Not so the Torah. It transmits great amount of material in few words. It demands and expects mental exertion from its readers. It assumes that they apply all of their imagination, learning and life experience to draw out this information. One way in which it accomplishes this is by intentional use of gaps.
We have seen an example of a gap in Midrash Shemini. That gap can be missed by an unskilled reader. The story of Cain and Hevel, however, contains such an obvious gap that it can only be intentional. As such it serves as a good illustration of gaps and how the midrashic method fills them.
Bareishis 4, 8.
And Cain said to Hevel, his brother. And it was at their being in the field and Cain rose upo against Hevel, his brother and killed him.
What is it that Cain said to Hevel? The gap yawns right before us .
How does midrash close gaps? In one of 3 ways. It is important to realize that a text that expects its reader toil and apply effort also expects that that reader read both forwards and backwards . The art of reading backwards is unfamiliar to us, drilled as we are by our elementary school teachers in the precise art of reading forwards. "Here is the introduction", they patiently taught us, "this is the first part, this one is the second and this is the conclusion." Let us see how these four ways realize themselves in this passage.
1. We look for an explanation in the story, characters or events that precede the narrative and shape our perception of what follows. This is classic reading forward, familiar to us from classic literature and cinematography where is sometimes called foreshadowing. In our case, it is the occupation of Cain and Hevel, sheep herding versus working the land, set out in verse 2.
What were they arging about? One said: "Let us divide the world. One will take land and one will take movable property." One said: "the land on which you stand is mine". The other one said: "What you are wearing is mine". This one says "Undress" and that one says "Fly in the air". From this discussion "Cain rose up against his brother and killed him."
2. We look for an explanation in what comes later. Now, when G-d confronts Cain, he attempts to fool Him and famously says "Am I my brother's keeper?" Apparently, Cain thought that G-d does not know and can be mislead. This leads Targum Yonasan to our verse to impute heretical views to Cain.
And Cain said to Hevel: "…There is no Judge and judgment and no World-to-Come and no reward for the righteous and no recompense for the wicked…
3. We look for an explanation from a distant context within Tanach itself, our understanding of the world and human character and behavior or, when available, from an orally transmitted tradition.
R. Yehishua from Siknin says…What did they argue about? This one said: The Temple shall be built in my portion" and that one said: The Temple shall be built in my portion. "
R. Yehuda Bar Ami says: They argued over the first Eve. R. Aivo said: First Eve returned to her dust . Rather they argued about the extra twin girl that was born with Hevel. This one said "I take her for I am firstborn. This one said "I take her for she was born with me."
Somewhere, someplace I had read a suggestion that the midrash is making an ironic comment on human nature for this is what brothers fight about most often - religion or a woman. If correct, this would be an example of using our knowledge of human nature to close the gap.
More likely, the midrash writes thusly because of the profound conviction that Cain and Hevel could not have argued over anything less than the location of the Temple . R. Aivo's opinion represents an ancient tradition regarding a twin sister that was the cause for Cain's murder of Hevel, attested to by such early works as Jubilees 4:1 but also found in Targum Yonasan to 4:2 .
The Midrashic explanations to this verse well illustrates the various approaches to closing intentional gaps and reminds us again to approach even the simplest appearing Biblical narratives with deep respect and a due measure of trepidation.
 As I recall both the Septuagint and Peshita add Cains's words at this juncture to close this intolerable gap; unfortunately, I do not currently have access to either translation. In any case, our focus in on the midrash and misrashic method and not on extra-rabbinic sources.
 The discovery that much of Biblical poetry is composed following a chiastic pattern - that is, in patterns of ABCB'A' practically demands recognition of this fact. The purpose of the chiasm is to draw attention to its center (C) where the key phrase or sentence pulses. As such it demonstrates the expectation of reading both forwards and backwards.
 Nechama Liebowitz suggested that the reason that Rashi refused to quote this midrash (there are midrashei aggada on this verse but this (my explanation is the proper interpretation of the verse)) because the verse suggests that Cain initiated the argument while the midrash implies that both were equally responsible. See S. Peerless, To study and to teach, Urim, 2004, p. 154
 As described in Genesis Rabba 18, 5. R. Yehuda's opinion is more difficult to understand. Perhaps, it hints to the creation of Lilith as the first Eve as described in mystical works.
 The widespread existence of certain interpretative traditions that are quoted by rabbinic and many non-rabbinic works, such as Septuagint, Jubilees, Dead Sea Scrolls, Philo and Josephus are an important testimony to the existence of generally accepted and widely shared oral tradition already in the beginning of the second Temple period. This point has been made by J. Kugel in the last chapter of his In Potiphar's House, Harvard University Press, 1990 as well as the more recent The Bible as It Was, Belknapp Press, 1997.