Sleep, kinsman thou to death and trance And madness, thou hast forged at last A night-long Present of the Past In which we went thro' summer France.
Hadst thou such credit with the soul? Then bring an opiate trebly strong, Drug down the blindfold sense of wrong That so my pleasure may be whole;
While now we talk as once we talk'd Of men and minds, the dust of change, The days that grow to something strange, In walking as of old we walk'd
Beside the river's wooded reach, The fortress, and the mountain ridge, The cataract flashing from the bridge, The breaker breaking on the beach.
Now Tennyson characterizes sleep not only as death, but also as insanity and "trance" (no—not the style of music, but, rather, kind of like being hypnotized).
In his dream, he's remembering a trip he took to France with his close friend and various things they did while there.
He wonders in the second stanza if Arthur has a kind of mystical power to recall this pleasurable trip to him while he's asleep. It's described as being a sort of drug-induced high caused by an "opiate."