This is a quote from Tim the Realist. I never looked at it this way, but I think it's brilliant.
Money is the one thing that we can send through time. By saving money, you are in effect sending it through time to your future self. By borrowing money, you are receiving money sent back through time from your future self, who will have to pay for it. The amount of money you can send through time depends on the amount of cash you currently have to send forward (save), and your current credit rating determines how much your future self can send back to you today.