How we cite our quotes:
Which by and by black night doth take away,
Death's second self, that seals up all in rest. (7-8)
On the whole, these lines seem to continue quatrain 2's emphasis on the grim inevitability of death. Of course, depending on how you read the word "rest" at the end of line 8, these lines could also be holding out a little sliver of hope about the afterlife. That's because the word "rest" might point to the traditional Christian idea of the dead people slumbering in their graves until Judgment Day comes around and they get up and ascend to Heaven (or descend to Hell). The problem with this interpretation is that, technically speaking, the word "rest" here refers to what "black night" brings on, and "black night" is described as being like "Death," but not Death itself (it is death's "second self"). Putting this analogy in SAT form, it looks like that "black night" is to "rest" as "Death" is to, well, "being dead." Yeah, maybe these lines really aren't that comforting after all.
In me thou seest the glowing of such fire,
That on the ashes of his youth doth lie,
As the death-bed whereon it must expire,
Consumed with that which it was nourished by. (9-12)
Quatrain 3 continues the death theme, but seems to shift the emphasis subtly. Instead of concentrating on Death as an external force coming to snuff out life, these lines put the emphasis on how life continues to endure—so that it only ends when it uses itself up. In a way, these lines can be read as the speaker thumbing his nose at mortality.
This thou perceiv'st, which makes thy love more strong,
To love that well which thou must leave ere long. (13-14)
The very end of the poem continues in the same emotional direction that was started in quatrain 3: defiance. Here, it's almost as if the speaker is saying, "Yeah, sure, Death, you think you're so tough. But you know what? That's just going to make me and my lover cherish each other all the more. Sucks to be you."