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the parts that persist

You've probably heard it: every cell in your body is replaced every seven years, so you're a completely different person than you were a decade ago. The real number, from a 2005 carbon-dating study, is a whole-body average — not a shared clock. Turnover is wildly uneven: gut lining renews in about five days, while cerebellar neurons, formed in early childhood, last a lifetime and never turn over at all. The tissue most tied to your sense of self is exactly the tissue that persists. That's a real problem for the idea that "you" are just your current atoms — but it's not evidence for anything beyond materialism either. Whether identity depends on matter at all is a question philosophers were already asking long before carbon dating existed.