What are we trying to achieve? It’s very simple actually — all we’re trying to do is maintain a simple table of balances on a piece of paper. This table will sh

What are we trying to achieve? It’s very simple actually — all we’re trying to do is maintain a simple table of balances on a piece of paper. This table will show how many coins each of our heroes has. The trick is, because we can’t have one piece of paper that holds the only source of truth — we’re going to have to keep things equal and let each of the gang maintain their own version — this is the decentralized part. And naturally, we’re also going to hope that all 4 pieces of paper eventually show the same thing — this is the consensus part. Nothing in the statement of the problem requires a block chain. Maintaining distributed ledgers is not a difficult problem except for one niggling detail: double spending. In other words, if the scenario you're looking at has no risk of double spending, a block chain isn't going to help you at all. To expand on the article's example... Jack is down to his last coin and gets a brilliant idea. He'll pay Kate and Hugo each one coin during the day. If he succeeds, Jack can forge one coin out of thin air. The islanders face a dilemma. Which of Jack's two payments should be accepted? Accepting the payment to Kate means that Hugo loses his. Accepting the payment to Hugo means that Kate loses hers. Accepting both payments is a non-starter. The islanders may as well just start bartering again if monetary scarcity can't be preserved. This is the motivating example behind block chains. The system we know as Bitcoin had been largely worked out by the time Satoshi released the white paper in 2008. Digital signatures and hash chains mean that forgery of coins or payments is practically impossible. What was missing was a way to make double spending difficult, without invoking a central authority.