10). The body is just a weak vessel that easily falls to the "knife" of a "coward" (line 11). The coward, obviously, suggests the personification of Time (or Death). And the knife suggests the image of the scythe mentioned in earlier sonnets. The point, though, is that the body is essentially worthless. It is not something that should be mourned.
In the final two lines, the couplet, the speaker places greater emphasis on the spirit of his being. He tells the young man that "the worth of that [his body] is that [his spirit] which it [his body] contains" (line 13). In other words, only his spirit has any value. The body has none. The speaker ends by telling the young man that "that [his spirit] is this [his sonnet]" (line 14). In other words, his spirit resides or is reflected in his poetry. Since the young man has his poems, the young man also has the spirit of the speaker. Thus, the young man should be glad that he still has the best part, the noblest part, of the speaker.
Although this poem uses the traditional Christian dichotomy (the division into two separate or opposite parts) of seeing a human being as containing both a body and soul, the poem itself is not actually expressing a Christian belief. The "spirit" that the speaker describes in this poem is not an actual soul that drifts upward to Heaven. Rather, it is a memory of the thoughts and feelings of the poet. The thoughts and feelings of the poet are captured in the poem. In that sense, then, the spirit of the poet is captured by the poem. Thus, the poem is his spirit.