Think back to when you were ten years old. Was that child you? Bear in mind that not only is virtually nothing physical about you the same now as it was then, you almost certainly have completely different preferences, beliefs, attitudes, goals, thoughts… even your personality will have changed. If that child was you, the same person you are now, what is it that accounts for this continuity in identity?
17th century English philosopher John Locke addresses this problem in his An Essay Concerning Human Understanding. He starts with an atom and declares that it is the “same with itself” at any instant of its existence. Although we now know that atoms aren’t fundamental particles, Locke would certainly have been referring to an irreducible, unchanging, fundamental particle of nature. Nowadays, we would reserve this title for quarks and electrons. Perhaps it would be better if we paraphrase Locke and say that a fundamental particle is the “same with itself” at any instant of its existence. Identity, at the level of fundamental particles, coincides with physical existence. The fact of existence ensures continuity of identity.