Money grows more complex by the day, particularly now that Silicon Valley has taken it up as one of its causes célèbres. Startups offer dozens of schemes to disrupt it. Yet, Silicon Valley, which has changed the world in so many fundamental ways, hasn’t actually changed money that much; it’s just evolved how we spend it. Paypal, Square, Stripe, Venmo—they’re all apps built on top of the old technology that is money. Just as paper once required metal, most payment apps require a bank account or a credit card.