The heart.
This poem by Simon Armitage sums it up well.
I've made out a will; I'm leaving myself
to the National Health. I'm sure they can use
the jellies and tubes and syrups and glues,
the web of nerves and veins, the loaf of brains,
and assortment of fillings and stitches and wounds,
blood - a gallon exactly of bilberry soup -
the chassis or cage or cathedral of bone;
but not the heart, they can leave that alone.
They can have the lot, the whole stock:
the loops and coils and sprockets and springs and rods,
the twines and cords and strands,
the face, the case, the cogs and the hands,
but not the pendulum, the ticker;
leave that where it stops or hangs.
The rest of the organs we visualise as little more than complex pieces of meat, the bits of the body that makes it all work. Think of the heart and your mind explodes; somehow, culturally, it has come to mean emotion, life, love, the soul. It's more than a part of the body, it's a part of the soul. It's everything that makes us human.
Speaking as the 16-year-old-me-studying-this-poem-for-my-GCSEs, what stands out about the poem is that he offers no explanation as to why the heart matters so much to him. And that, I think, is the point. Something about the way we perceive the heart cannot be put into words. But simply put, it matters.