You replaced a belief last Tuesday. A small one. You read something, and the way you understood a thing shifted. Slightly. You didn't announce it. You didn't notice the moment it happened. But the you who woke up Wednesday is not identical to the you who went to sleep Monday. Was that growth? Or did something else move in while you weren't looking? How would you know? The physics of identity is the physics of trust. And two thousand years of philosophy have been arguing about planks without one measurement instrument.
An Icelandic parliamentarian, sitting in the Althing -- one of the oldest continuous democratic assemblies on earth, founded a thousand years before the first database -- heard this argument over coffee and said: "This is the heart and soul of AI." He is a musician. He heard the note before I could name it.
You give: the assumption that you're the same person you were last year. You get: a way to check. Not philosophically. Physically. The instrument exists. The number is real. And the stakes are not academic. They are the most personal thing there is: whether the thing making your decisions is still you.