What happens when your health issues take center stage, forcing you to reschedule tours, delay plans, rearrange your artistic desires? What happens when your mind races ahead but your body stays put? What happens when you start to feel like everyone knows you as "that rapper who got the kidney transplant"? What if you feel sick and healthy at the same time? What if you are P.O.S, and you're trying to pinpoint the perfect way to sum up three years of witnessing the best and worst of humanity and your own body's abilities, knowing that this will be the first thing you've said in an awfully long time?

You might end up rapping about quantum physics. You might invite all your friends to help out. And it just might end up being one of the finest songs of your career.

"All I want is to chisel my initials into something permanent now, and raise up these damn kids and make my mama so f***ing proud, and mutilate a couple crowds," P.O.S spits about three minutes into the nine-minute "Sleepdrone/Superposition." It's an urgent statement that comes amid a flurry of cameos from rappers Lizzo, Astronautalis, Allan Kingdom, Hard_r (P.O.S's son, Jake), plus riot grrrl founder Kathleen Hanna (!), Eric Mayson, Lydia Liza, and Nicholas L. Perez, all of whom manage to stay mostly out of the way as P.O.S ruminates on his health, relationships, and identity.

"I'm trying to exist in superposition," he insists repeatedly. For such an epic song, he's invoking an equally epic quantum theory; superposition is the idea that a being can exist in multiple states at once, a la Schrodinger's cat (who gets a shout-out in the song). On "Sleepdrone/Superposition," P.O.S is a black man confronting police brutality and coming to terms with the deaths of Eric Garner and Mike Brown; a father grappling with his relationship with his kids and his own parents; a surgical patient trying to overcome chronic pain; and a rapper so lyrically gifted and intellectually complex that he seems to rise above it all and float untethered over the mayhem.

It's ambitious as all hell. And yet somehow, the whole song manages to fly by in an instant, despite the fact that it's the length of three pop songs. It's the kind of opus that demands repeat listens, if only to catch all the guest appearances and lyrical turns of phrase. And it's so, so good to have P.O.S back in action.