Cash was private by default. You handed it over, the other person had it, and that was the end of the record. For most of history that is how money behaved.
Bitcoin works differently, and it is worth knowing why. Every payment is written to a ledger, which is a shared record of transactions that anyone can download and read. It is public on purpose. That is how you can check that only twenty one million coins will ever exist without asking anyone to vouch for it. You verify it yourself.
The cost is that the record never forgets. Pay your barber in bitcoin and he can see the balance of the address you paid from. He can see what you did with it afterward. He can look again next year.
I keep coming back to one thing. Privacy in money has to be part of the design from the first day, because a public record cannot be un-published later. You can add a privacy tool to a network that has been open for a decade, and plenty of people have. What you cannot do is go back and remove ten years of history already sitting on thousands of computers.
The strongest argument against me is a good one and I want to state it fairly. That same public record is what makes Bitcoin checkable. Privacy techniques make auditing harder, and they do make life easier for people doing real harm. Anyone who waves that away is selling something.
What would change my mind: a network that spent years fully open, added privacy afterward, and got most of its ordinary users onto it, with the supply still checkable by anyone. If that happens, bolting it on works and I am wrong.
I have not seen it yet. What I see instead is that optional privacy stays a minority setting, and a minority setting makes its users stand out. The people who need protection the most end up the easiest to spot.
Cash never asked you to opt in. That is the bar.
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