We Might As Well Be Chilling With Hamlet in the Graveyard
Even though the speaker addresses something nobody ever gets to see (his "soul" or, his immortal spirit), he uses a lot of images from the physical world to make the following point: our physical bodies are going to die but, our souls can live forever. We talk about these images more in "Symbols, Imagery, Wordplay," but our personal favorite has got to be the terrifying image of worms feeding on corpses.
Why's that? Because it transports us to a creepy graveyard where bodies are buried in the ground, that's why. When the speaker asks his soul "Shall worms, inheritors of this excess / Eat up thy charge?" (7-8), our answer is a terrifying "Yes." Unless we've got big plans for cryopreservation or a Viking Funeral or something.
Reading this sonnet sort of makes us feel as though we're kicking it in the graveyard with Hamlet, who holds up that famous skull and looks death in the face. That's basically what the speaker of Sonnet 146 forces us to do—acknowledge the certainty of death. Gee. Sounds like a fun guy to hang out with at parties, right? But to be fair, the speaker does offer up some hope when suggests that our souls can kick it in heaven forever. So, there's that.