The Torah discusses Reuven's actions with Bilhah in but half a verse:
וַיְהִי בִּשְׁכֹּן יִשְׂרָאֵל בָּאָרֶץ הַהִוא וַיֵּלֶךְ רְאוּבֵן וַיִּשְׁכַּב אֶת בִּלְהָה פִּילֶגֶשׁ אָבִיו וַיִּשְׁמַע יִשְׂרָאֵל וַיִּהְיוּ בְנֵי יַעֲקֹב שְׁנֵים עָשָׂר.
And it came to pass, while Israel dwelt in that land, that Reuben went and lay with Bilhah his father's concubine; and Israel heard of it. Now the sons of Jacob were twelve:
These words reveal almost nothing of the incident, sharing only the bare minimum: that Reuven had relations with his father's concubine. They discuss neither Reuven's motives nor what Yaakov did after hearing of the fact. What prompted Reuven, ostensibly a righteous figure, to commit such a deed? How is Yaakov's reaction to be interpreted? Finally, why does the Torah decide to end the episode with the statement, "וַיִּהְיוּ בְנֵי יַעֲקֹב שְׁנֵים עָשָׂר"? Is this fact somehow relevant to Reuven's actions, or should it be viewed as an independent unit, disconnected from our story?1
In contrast to Yaakov's silence here, on his deathbed he does chastise Reuven, saying:
(ג) רְאוּבֵן בְּכֹרִי אַתָּה כֹּחִי וְרֵאשִׁית אוֹנִי יֶתֶר שְׂאֵת וְיֶתֶר עָז. (ד) פַּחַז כַּמַּיִם אַל תּוֹתַר כִּי עָלִיתָ מִשְׁכְּבֵי אָבִיךָ אָז חִלַּלְתָּ יְצוּעִי עָלָה.
(3) Reuben, thou art my first-born, My might, and the first-fruits of my strength; The excellency of dignity, and the excellency of power. (4) Unstable as water, have not thou the excellency; Because thou wentest up to thy father's bed; Then defiledst thou it — he went up to my couch.
The phrase, "כִּי עָלִיתָ מִשְׁכְּבֵי אָבִיךָ" would seem to be a clear reference to Reuven's actions with Bilhah. The rest of Yaakov's words, though, are more ambiguous. What does "יֶתֶר שְׂאֵת וְיֶתֶר עָז" mean? Who is the referent of the verb "עָלָה" in the phrase, "אָז חִלַּלְתָּ יְצוּעִי עָלָה"? What light can this whole passage shed on the initial affair?
PunishmentElsewhere, too, there is an allusion to the episode. Divrei HaYamim I 5:1 shares:
(א) וּבְנֵי רְאוּבֵן בְּכוֹר יִשְׂרָאֵל כִּי הוּא הַבְּכוֹר וּבְחַלְּלוֹ יְצוּעֵי אָבִיו נִתְּנָה בְּכֹרָתוֹ לִבְנֵי יוֹסֵף בֶּן יִשְׂרָאֵל וְלֹא לְהִתְיַחֵשׂ לַבְּכֹרָה. (ב) כִּי יְהוּדָה גָּבַר בְּאֶחָיו וּלְנָגִיד מִמֶּנּוּ וְהַבְּכֹרָה לְיוֹסֵף.
(1) And the sons of Reuben the first-born of Israel — for he was the first-born; but, forasmuch as he defiled his father's couch, his birthright was given unto the sons of Joseph the son of Israel, yet not so that he was to be reckoned in the genealogy as first-born. (2) For Judah prevailed above his brethren, and of him came he that is the prince; but the birthright was Joseph's —
According to this passage, Reuven did actually get punished for his sin, losing his birthright to Yosef and kingship to Yehuda. Why is this specific punishment chosen? Why is it not mentioned in Bereshit 35?