Sustainability sounds nice, but it’s extremely hard to pin down a definition. To quote “Doctor Who,” sustainability is a bit like “wibbly wobbly … time-y wimey … stuff.”

But in all seriousness, the word sustainability gets thrown around all over the place without anyone really agreeing on what we’re talking about, which made for one heck of a conversation at the University of Iowa Public Policy Center’s forum Tuesday evening on sustainable economics.

Aaron Strong, a UI assistant professor of urban and regional planning who studies environmental economics, explained sustainability with a Venn diagram. Ideally, it’s where economics is in balance with the environment and equity.

The million-dollar unanswerable questions that everyone asks are “How do we get to a sustainable economy?” and “What will that actually look like?”

The problem with this conversation about sustainability is that there’s no agreed-upon definition, much less any sort of consensus on how to go about addressing resource depletion, climate change, income inequality, and a whole slew of other factors that are involved in the debate surrounding sustainability.

Most people seem positively convinced that a big change of some sort is coming, but what it means, no one knows. Does it mean the end of economic growth? Would that bring technological innovation to an end? Would it mean we could all work less and pursue our personal passions and launch a journey of self-discovery?

Depends on who you ask.

The general idea, as the Public Policy Center’s panel outlined, is that society has to agree on what’s in its best interests and do that. But as Strong pointed out, “We don’t agree as a society what’s best for society.”

That becomes extremely problematic when we are forced to make sacrifices and hard choices, which, pretty much everyone agrees, is going to happen. For example, much of the U.S. Southwest is having huge problems with recurring droughts. This is putting bigger and bigger strains on those resources. Should we let people continue to move there? Should we use incentives to get them to go elsewhere or just let them do their thing?

At what point does what’s good for society trump individual liberty?

We’re obviously going to hit crunch time, and that’s probably when we’ll act. But will that require a massive reorganization of society? Or can scientists simply help us innovate our way out of the problem?

No one knows, and hardly anyone agrees.

We simply don’t know the exact costs that climate change and environmental degradation will impose, so we don’t have any idea what to do.

That’s what makes this conversation so hard — we just don’t know.

And that’s enough to get people riled up, which is totally understandable. Planning for the future really helps relieve anxiety about what’s going to happen tomorrow, next month, or 10 years from now. So when you can’t plan, when you’re absolutely convinced that our society is going to drastically change in the near future, that’s enough to freak people out.

Maybe we just have to get used to not knowing. In the United States especially, we’ve grown accustomed to the luxury of knowing that we’re safe, that when we wake up tomorrow, most everything in our daily lives will be the same as it was before.

Whatever happens, we’re going to have to figure out how to resolve this oncoming storm sooner or later, and the sooner we get our act together and start planning, the better.