Genesis 49:4

4.Unstable as water. He shows that the honor which had not a good conscience for its keeper, was not firm but evanescent; and thus he rejects Reuben from the primogeniture. He declares the cause, lest Reuben should complain that he was punished when innocent: for it was also of great consequence, in this affair, that he should be convinced of his fault, lest his punishment should not be attended with profit. 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 ascribed to anger, as if the father desired to take private vengeance of his son: but it proceeded from the Spirit of God; because Jacob kept fully in mind the burden imposed upon him. The word עלח (alach) the close of the sentence signifies to depart, or to be blown away like the ascending smoke, which is dispersed. (196) 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 expound the expression concerning the bed, to mean that it ceased to be Jacob’s conjugal bed, because Bilhah had been divorced, is too frigid.