Commentary for Genesis 49:4

פחז כמים The impetuosity and the precipitance with which you so hastily showed your wrath, just like water that rushes headlong in its course — therefore

UNSTABLE (‘PACHAZ’) AS WATER, YOU HAVE NOT THE EXCELLENCY. Pachaz is a noun, derived from the expression, Vain and reckless (‘upachazim’) fellows,⁸⁰Judges 9:4. 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,”⁸¹Nedarim 9b. that is “my evil inclination leaped upon me” They said further: “It is their recklessness (apichzeihu) which they are revealing,”⁸²Sanhedrin 57a. See Note 63 in my Hebrew commentary, p. 265. meaning their impulsiveness. “A reckless (p’chizah) people, who spoke before listening, you still retain your recklessness (‘b’pachzuteich’),”⁸³Shabbath 88a. meaning “hasty and impulsive.” It is possible that this word pachaz is a permutation of the word chipazon (hurry).⁸⁴As is the case with the words; keves 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,⁸⁵The Hebrew is mishk’vei (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.⁸⁶Isaiah 8:7-8.
Our Rabbis have said:⁸⁷Shabbath 55b. It is found here in Rashi in the form mentioned by Ramban. “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,⁸⁸I Chronicles 5:1. This clearly shows that it was “the couch” which was defiled and not, as the comment of Rashi would have it, the Shechina. 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.⁸⁹Psalms 132:3. Thus Jacob’s intent is to say: “Then thou didst defile me;” He spoke in the third person⁹⁰Alah, “he” went up. only as a matter of respect. Similarly, the verse, Forasmuch as he defiled his father’s couch,⁹¹I Chronicles 5:1. is a Scriptural modification, meaning “forasmuch as he [Reuben] defiled the one who goes up⁹²I.e., Jacob. His name though is not mentioned directly, as a matter of respect. upon his father’s couch.”
From the literal meaning of this verse it would appear as I have explained it in Seder Vayishlach,⁹³Above, 35:22. 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.”