If you have a surplus in your household budget, what’s the best use for it? Does it make more sense to pay down debt or to invest those extra funds? With interest rates at such low levels, this is a question I’ve been hearing with increasing frequency.

Implicit in this question is a straightforward assumption—that you’ll earn more investing than you would save by paying down debt. Suppose, for example, your mortgage is at 3.5%. If you pay down debt at that rate, that’s equivalent to gaining 3.5%. Alternatively, if you invested in the stock market, your return would be uncertain. You might earn more than that, but there’s also the possibility you could earn less, with this year being a good case in point. Historically, though, the U.S. market has gained 10%, on average. So even if the market did just half as well in the future, you’d still be doing better than 3.5%. Based on this logic, then, it seems like there’s an easy answer to this question: The rational choice would be to invest any extra money you have.

That is the mathematical answer—and it’s hard to dispute—but I don’t think it’s the only answer. Instead, I see this as a question with a lot of nuance. I would also consider the following:

Taxes. If you’re doing a calculation like the one above, be sure to adjust for taxes. Most debt doesn’t carry a tax deduction, but mortgage debt does (up to $750,000, or $1 million for purchases prior to 2018). That means that a 3.5% mortgage might be costing you little more than 2%—or even less, if you’re in a high tax bracket and/or live in a higher-tax state like California. Similarly, every investment has a different tax profile: Some stocks issue dividends while others do not. And in the bond world, some pay interest that is subject to tax while others do not. Bottom line: If you are doing a comparison, be sure to compare apples to apples.

Leverage. When I was in school, I remember learning an economic theory called the Modigliani-Miller Theorem, which stated that companies should be agnostic as to how they raise money. Whether they issue shares or take on debt, it shouldn’t make a difference, even if that means the company ends up highly indebted. When I first heard this concept, I thought it was crazy, and I still do. That’s because debt matters. To put it in simple terms, companies with no debt can’t go bankrupt. The same is true of individuals and families. During good times, debt may be very manageable. But this year is a good example of how life can throw financial curveballs—or screwballs. So even when the math says that you’re better off investing, it’s worth reviewing your overall balance sheet and asking whether it would, nonetheless, be worthwhile to reduce your debt load.

Liquidity. It’s important to think about your overall debt load, but it’s also important to think about the relationship between indebtedness and liquidity. If you pay down a dollar of debt today, that will give you more flexibility in a rainy day scenario. But if you keep that same dollar in the bank, you’ll have more flexibility for other things. That might include an unexpected expense or maybe an investment opportunity. There is no scientific way to strike this balance, but a good rule of thumb is to avoid going to either extreme.

Personal preference. Let me describe two people I know: I’ll call them Joe and Sam. Joe has lived in the same house for 20 years, and yet he has virtually no home equity. This is intentional, and he is proud of this fact. Over the years, Joe has refinanced his mortgage more than ten times, often taking money out. Joe’s view: With rates so low, he’s happy to continue rolling over this debt, and this gives him the flexibility to use his money in other ways. He plans to do this as long as he can. In his words, “If I don’t ever have to pay for my house, why should I?” Now let’s look at Sam. He has also lived in the same house for 20 years. He started with a 30-year mortgage but has been making extra payments all along and is nearly done paying it off. Who’s right here—Joe or Sam? To be sure, you could do the math and maybe prove that one or the other made the better choice. But there’s something else going on, and that’s personal preference. Some people just don’t mind living with large amounts of debt while others are the opposite—they dream of not owing a penny to anyone. Most people fall somewhere in the middle. But if you’re clearly a Joe or a Sam, the math might not matter as much as your personal preference—and I think that’s perfectly okay. Personal finance isn’t all about optimizing every dollar; it’s about optimizing happiness and peace of mind. To be sure, you shouldn’t make choices that jeopardize your financial security, but if you’re considering two reasonable choices, you don’t always need to do what the math says is “best.”

Market valuation. I often talk about the dangers of market timing—trying to predict whether the market is going to go up or down. But there’s a big difference between trying to predict where the market is going and simply reacting to what the market has already done. If you have a surplus in your budget, you might opt to invest more when the market is down than when it’s flying high. You could do this formulaically, tying the size your investments to the level of the S&P 500, for example, and investing more when the market is down. Or you could do it informally. But since you’re making relative judgments on uses of funds, it’s important to compare both sides of the equation. Depending on where the stock market stands, it may be relatively more or less attractive, and you should invest accordingly.