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Foreign Hi, I'm Andy Tempte, and welcome to the Saturday Morning Muse. Start your weekend with musings that are designed to improve financial literacy around the world. Today is August 2, 2025. Over the past few months, we've explored the origins of global trade, traced the evolution of money, and we've seen how trust became the bedrock of banking. Last time, we discovered how bank banknotes evolved as promises, or IOUs, that revolutionized commerce by making money portable and scalable. But here's the thing about those promises. Those IOUs, they are not free. When I lend you my car for the weekend, I can't use it. When a bank lends you money to buy a home, that capital sits locked up for up to 30 years. This brings us to one of the most fundamental and historically contentious concepts in economics, the concept of interest. Well, but what is interest? At its core, interest is the price of time. More specifically, it's compensation for allowing someone else to use your money today instead of you using it yourself. Think of it this way. If I give you $100 today, you get the benefit of spending or investing that hundred dollars right now, but I lose the opportunity to spend or invest that cash. Interest is what you pay me for that privilege. But there's a deeper layer here that connects directly to our June discussion about inflation. Remember how we expl explored how the purchasing power of money erodes over time? That same $100 I lend you today will buy less stuff when you pay me back next year. If inflation is running at, let's say, 3%. So interest isn't just about opportunity cost. It's about preserving purchasing power. So this is what economists call the time value of money. A dollar today is worth more than a dollar tomorrow because of three realities. Inflation erodes purchasing power. That's number one. Number two, money can be invested to earn a real return. The real rate of return. That's number two. And number three, there's always risk that a dollar invested today might not be recouped in full or at all. You might lose the whole thing. So here's where we get practical. For any lender, whether it's your neighbor, a Community bank, or JPMorgan Chase, the interest rate that they charge must cover three things. One, expected inflation. If I expect prices to rise 3% over the next year, I need at least 3% interest just to break even. In purchasing power terms, this is called the inflation premium. Second, I need a real rate of return. If I'm giving up the use of my money, I deserve compensation beyond just Maintaining purchasing power. Maybe I want a 2% real rate of return for my trouble. This is called the real risk free rate. And third, there is risk compensation. There's always a chance that you won't pay me back. The riskier the loan, the higher this premium needs to be. Maybe I need 4% to compensate for that risk. We call this the risk premium. If I add all those up, 3% inflation, 2% real return and a 4% risk premium, that would mean that I would charge you a 9% interest rate to loan you money today. Those are the basics. How interest rates are determined irrespective of the situation. Now the question is, how is interest calculated? So setting the interest rate is one thing. Calculating interest, the actual amount that you pay is a little bit different. And today we're going to start with the most basic method of how interest is computed. And this is called simple interest. This represents the most fundamental way to calculate interest. Though in future muses we're going to explore the more powerful concept of compounding. And we'll do that in detail. So wait for that one. The formula for simple interest is very straightforward. It is the principal value of the loan, the amount that's lended times the interest rate. That we've already discussed how that's determined times the amount of time that is transpiring. So if I lend you $1,000 at 5% simple interest for two years, here's the math. Interest equals 1,000 times 0.05 or that 5% times two or two years and that equals $100. So in two years time, you'll pay me back $1,100 total, which is the original $1,000 of principal plus the $100 in interest. Note that simple interest is linear. It does not compound over time. Each year you pay interest only on the original principal amount. If you borrowed that $1,000 for 10 years at 5% simple interest, you'd pay $50 in interest per year, totaling $500 in interest over that 10 year loan period. Now, I learned personally about interest the hard way, as most of us do. My first bank loan at age 19 was for lighting equipment for my rock band. And since the collateral was professional lighting gear for of young adults with musical aspirations, the risk premium was quite high. But I learned later that the banker wasn't being greedy. They were running a business that had to account for the cost of money, the risk of default and the erosion of purchasing power. When I finally understood this, I stopped seeing interest as punishment and started seeing it as the price of accessing. Opportunity today. And we're going to talk a lot more in the future about the benefits and risks of taking on loans and debt. When should you use debt? And when should you try to save up all the cash and just use cash to make a purchase? Now, understanding interest is not academic. It is survival. In our modern economy. Every mortgage payment, every credit card statement, every business loan represents these same principles that we've outlined about how interest rates are determined and how interest is calculated. When you see a 6% mortgage rate, you're seeing the market's collective judgment about inflation expectations, risk levels, and the real time value of money or that real return that we discussed. More importantly, understanding interest helps you make better decisions. Should you pay off your 3% mortgage early or or invest that money in the stock market that might be earning a higher return? Should you take out a business loan to expand operations? Should you finance that car or wait and pay cash? You can't answer these questions intelligently without understanding what interest really represents. Interest rates are indeed the price signals that coordinate our entire economic system. They guide resources from savers to borrowers, from those with excess capital to those with productive opportunities. When interest rates are low, borrowing is cheap, encouraging businesses to expand and consumers to make major purchases. When interest rates are high, the price of money is high. Savers earn attractive returns on deposits and bonds, while borrowers face expensive financing costs that discourage spending and encourage saving instead. This is why central banks, like our Federal Reserve in the United States, pay such close attention to interest rates. They're not just numbers on a screen. They're the steering wheel for the economy. But here's what makes the story of interest truly fascinating. For most of human history, charging interest was considered morally wrong. Ancient civilizations, major religions, and entire legal systems prohibited or severely restricted interest. Yet economic reality kept pushing back because commerce demanded credit, growth required investment, and trade needed financing. So next week, we're going to explore this tension, how civilizations from ancient Babylon to medieval Europe grappled with the moral implications of charging for the use of money. We'll see how clever merchants found workarounds, how religious scholars debated exceptions to the rule, and how practical necessity ultimately shaped our modern understanding of interest. For now, the key insight is this Interest isn't arbitrary. It's not banks being greedy. It's the market's way of pricing time, risk, and opportunity. Understanding this makes you a more informed borrower, a better saver, and a wiser participant in our complicated financial world. So until next time, I wish you grace, dignity and compassion. My name is Andy Tempte. This is the Saturday Morning Muse. You can find the show on all the major streaming services as well as out on YouTube. Please, like, subscribe, rate and most importantly, share this public good with your friends, your colleagues, your neighbor, and maybe a family member or two. The show was produced by Nicholas Tempte, and we'll see you next time on the Saturday Morning Muse.