Genèse 49:4

I will liken thee to a little garden in the midst of which there enter torrents swift and strong, which it cannot bear, but is overwhelmed. Be repentant then, Reuben my son, for thou hast sinned, and add not; that wherein thou hast sinned it may be forgiven thee; for it is reckoned to thee as if thou wentest in to have to do with the wife of thy father at the time that thou didst confound my bed upon which thou wentest up.

UNSTABLE (‘PACHAZ’) AS WATER, YOU HAVE NOT THE EXCELLENCY. Pachaz is a noun, derived from the expression, Vain and reckless (‘upachazim’) fellows, meaning “hasty and impulsive people.” Our Rabbis have made steady use of this word: “I was looking at my image in the water and my evil inclination rose (pachaz) within me,” that is “my evil inclination leaped upon me” They said further: “It is their recklessness (apichzeihu) which they are revealing,” meaning their impulsiveness. “A reckless (p’chizah) people, who spoke before listening, you still retain your recklessness (‘b’pachzuteich’),” meaning “hasty and impulsive.” It is possible that this word pachaz is a permutation of the word chipazon (hurry). and kesev, both of which mean “lamb,” and other similar words. Jacob was thus saying: “Since you burst forth as water, you shall no longer excel for thou wentest up to thy father’s place of repose in thy haste and impulsiveness. Then didst thou profane them, (places of repose), which is in the plural form. Hence Ramban writes otham (them). when your recklessness ascended my couch like the rising and gushing water.” This is similar to the Scriptural description of water: Now, therefore, behold, the Lord bringeth up upon them the waters of the river, mighty and many, even the king of Assyria and all his glory; and he shall come up over all his channels, and go over all his banks; and he shall sweep through Judah, overflowing as he passeth through.
Our Rabbis have said: “Then didst thou profane He Who hovered over my couch, meaning the Shechinah” that used to abide above my [Jacob’s] couch. Yet the verse says, Forasmuch as he [Reuben] defiled his father’s couch,. Ramban proceeds to answer that Scripture modifies the expression out of respect for the Divine Presence. thus clearly stating that it was the couch which was defiled! But perhaps [out of respect for the Divine Presence, Scripture] modifies the expression there.
But if [we accept the explanation that Scripture modifies the expression], it is possible that the word alah (going up) refers to [Jacob, thus saying, “then didst thou profane my couch upon which I used to go up],” just as in the verse, Nor go up into the bed that is spread for me. Thus Jacob’s intent is to say: “Then thou didst defile me;” He spoke in the third person, “he” went up. only as a matter of respect. Similarly, the verse, Forasmuch as he defiled his father’s couch, is a Scriptural modification, meaning “forasmuch as he [Reuben] defiled the one who goes up upon his father’s couch.”
From the literal meaning of this verse it would appear as I have explained it in Seder Vayishlach, i.e., that it was Reuben’s intention to disqualify Bilhah from his father so that she should no longer give birth to children, thus lessening his share as the firstborn. It was for this reason that Jacob said to him that it was reckless and impulsive to think that he would gain thereby, whereas he had no profit from it but only loss.

כי עלית על משכבי אביך, for through your having mounted your father’s bed, (you disgraced, etc.). Reuven having been deprived of his privileges as first-born has been spelled out more clearly in Chronicles I 5,1-2) “He was the firstborn of Israel; but when he defiled his father’s bed, his birthright was given to the sons of Joseph, son of Israel, so he is not considered as first-born in genealogy.”

יצועי עלה, the indirect mode עלה, “he mounted,” means “you were about to mount.” We find numerous instances in Scripture when a past tense is not really a past tense, such as when Avraham said to Efron that he had already given the money for the cave of Machpelah, נתתי כסף השדה קח ממני, when what he meant was that he had the money all ready and prepared. (compare Genesis 23,13). [Avraham could not have paid for the field at that time, not having wanted to buy the field nor even having made an offer to buy it. He would have considered it improper to ask someone to sell an ancestral field. Ed.]

אז חללת יצועי עלה, at that time you defiled your father’s dignity. Alternatively, Yaakov may have referred to Reuven’s act being a desecration of G’d’s honour. As an appropriate punishment, Reuven’s own status, which had been one of special honour, would be diminished accordingly.

כי עלית משכבי אביך, when you committed that act, חללת יצועי, you defiled my couch; your act prevented me from having any further marital relations with my wife Bilhah. My grandfather explained the expression יתר שאת as an allusion to atonement. [שאת from the root נשא, to elevate, i.e. to forgive as in ונשאתי לכל המקום, “I will forgive the whole place” (Genesis 18,26) Ed.] If so, far from putting down Reuven in his final words to him, Yaakov blessed him by offering his forgiveness at this time for the sin he had committed. against him.

כי עלית על משכבי אביך, “you went up to your father’s bed;” On which occasion did this take place? At the time when Leah traded Yaakov’s turn to spend the night with Rachel for the mandrakes her son Reuven had found. (Genesis 30,15). (Br’eshit Rabbah 96.4)

חללת יצועי עלה, “you did defile Him Who used to ascend my couch. (The Holy Spirit)” Yaakov’s curse was to remain in force until Moses Deuteronomy 33,6 said: יחי ראובן ואל ימות, “may Reuven live and not die.” In the meantime it had been Moses who was allowed to ascend to G–d’s residence on the Mount; (Exodus ומשה עלה 19,3). An alternate interpretation of this phrase: “you have desecrated my couch which until that time had been on a higher level than that of my father and grandfather, each of whom had had a son that was not loyal to the Jewish tradition.” Yaakov, as if speaking about a third person, when he said משכבי אביך, “the couch of your father,” instead of: “my couch,” repeated this method when saying יצועי עלה, “he ascended my bed.”