Genesis 49:4

Unstable as water, thou shalt not excel; because thou wentest up to thy father's bed; then defiledst thou it: he went up to my couch.

Unstable as water - a boiling up as of water (art thou); i:e., thou didst boil up with lust and passion-referring to his incest (Gesenius).

Thou shalt not excel - i:e., thou shalt not preserve thy natural excellency in thy posterity, nor have the preeminence of rule. The criminal was degraded, but not otherwise punished personally. But he was in his tribe; because his descendants never made any figure:-no judge, prophet, nor ruler sprang up from among them, and the tribe of Reuben, together with the other transjordanic tribes, was the first that was carried into captivity (1 Chronicles 5:26).

Thou wentest up to thy father's bed - the bed being spread upon a divan, which itself is raised somewhat from the floor.

He went up to my couch. The third person is used here as if, instead of addressing Reuben directly, the indignant patriarch were pointing him out with loathing to his other sons. The only instance of this tribe trying to regain its lost primogeniture was the defeated attempt at rebellion narrated in Numbers 16:1-50.