Spoilers for Steven Universe
Steven Universe is filled with Gems. Throughout the show, Gems insist that their bodies are simply a conscious manifestation of light, whereas their gems are what they truly are. Their bodies are material and replaceable, and their gems are their souls. A body without a Gem is simply light while a Gem without a body is still that Gem.
Many philosophical and religious writings have examined this separation between soul and body. Descartes examined this concept and named it dualism (his specific interpretation was later named Cartesian dualism). Descartes theorized that the soul and the body were separate entities. While the body could be separated, the spirit could not.
However, the Gems take this a step farther than Descartes. Their souls are physical. Gems can be corrupted and even divided in the show. Shards of gems create clusters: artificial, mutated Gem fusions. This goes against Descartes’s interpretation which states that the soul cannot be divided. The show could also be taking this concept a step farther in making the concept of body even more alien. Rather than seeing the body as a conscious manifestation of light, here, the physical gem is the body, or at least another, separate representation of the body. This would also explain why most Gems think that Steven is Rose Quartz in disguise.
Gem clusters contain an amalgamation of body parts. It seems that parts of the gem correspond to hands, feet, legs, etc. So, it would stand to reason that there is an inherent connection between the soul and body. That, however, brings many questions about Gems and immortality. If the clusters still hold the souls of the dead gems, does every shard hold the soul of the Gem? If this is the case, do Gems ever really die, or are they simply trapped for eternity?
As the show progresses, The Crystal Gems learn more about Steven’s abilities as a human and a Gem. Unlike the Gems, Steven can age, though he is more or less able to control the aging process. He is also more fragile than the Gems and performs basic human biological functions. By becoming his caretakers, the Gems must face hard realities about Steven’s differences from the rest of the Gems. If he were to be harmed, they face losing Rose Quartz as well as Steven. However, Steven, like Connie, his father, and the rest of the humans in the show, may not be tied to his and Rose’s gem in the same way as the other Gems. And, even more odd, the Crystal Gems often refer to Rose as if she has died even though her gem is still intact. They do not refer, however, to Ruby or Sapphire in this way. There is a loss felt with Rose that is absent for Ruby or Sapphire.
Ultimately, it seems that Gems are based on a sort of dualism, but their alienness causes them to deviate from Cartesian dualism. Here, there is a greater distinction between their body (made of light) and their soul (made of gem), but the soul has a physicality that can be divided. Still, the show is more or less about the contrast between human and gem culture. Steven, as well as Connie, are being trained by the Crystal Gems to fight for earth. For Steven, there is a chance that his death would be impermanent. For Connie, however, this is not the case. Connie is being trained by Pearl, who we see lose her physical form many times protecting Rose Quartz. However, unlike Pearl, Connie has no physical manifestation of her soul to recreate a body. If her body were to be destroyed, then perhaps Descartes’s theories would apply to her as a human.