This is an article about Open Source Finance. It's an idea I first sketched out at a talk I gave at the Open Data Institute in London. By 'Open Source Finance', I don't just mean open source software programmes. Rather, I'm referring to something much deeper and broader. It's a way of framing an overall change we might want to see in the financial system. To illustrate this, I set up an analogy between computer systems and economic systems, and I then explore what financial 'code' might be. I then sketch out the five pillars that could underpin an open finance movement.