John Calvin Commentary Genesis 49:4

"Boiling over as water, thou shalt not have the pre-eminence; Because thou wentest up to thy father`s bed; Then defiledst thou it: he went up to my couch." — Genesis 49:4 (ASV)

Unstable as water. He shows that the honor that did not have a good conscience as its guardian was not firm but evanescent; and thus he rejects Reuben from the primogeniture. He declares the cause, so that Reuben would not complain that he was punished when innocent. For it was also very important in this matter that he be convinced of his fault, so that his punishment would not be without benefit.

We now see Jacob, having laid carnal affection aside, executing the office of a prophet with vigor and magnanimity. For this judgment is not to be attributed to anger, as if the father desired to take private vengeance on his son; rather, it proceeded from the Spirit of God, because Jacob was fully mindful of the burden imposed upon him.

The word עלח (alach) at the close of the sentence signifies to depart, or to be blown away like the ascending smoke, which is dispersed. Therefore, the sense is that the excellency of Reuben, from the time that he had defiled his father’s bed, had flowed away and become extinct. For to interpret the expression concerning the bed to mean that it ceased to be Jacob’s conjugal bed because Bilhah had been divorced is too unconvincing.