It’s a strange time to be an environmentalist.

Over the years, the once-cohesive environmental community has divided into multiple factions, splitting along philosophical and tactical lines. Some environmentalists, such as Arne Naess, have advocated a “deep ecology” view, which retreats from the modern world, seeking ways to live in harmony with the nature. Others, such as Bill McKibben and his group, are focused on creating a widespread political and social movement, reminiscent of the 1960s and ’70s, to rattle cages and change things through protests and grassroots action. Then there are the wonks and scientists, such as most of us in academia and government, who typically focus on the role of science and policy in shaping the future of the environment.

Others, like Bjorn Lomborg and the Copenhagen Consensus group, challenge traditional environmentalism and call for different international priorities, mainly focused on economic development, poverty alleviation and public health.  Also challenging the environmental mainstream are Michael Shellenberger, Ted Nordhaus and the Breakthrough Institute, who focus on the power of innovation and technological change to solve big environmental problems.

Once divided, the debates start. Whether about nuclear power, the use of genetically modified crops, the Keystone pipeline, the importance of climate change or the role of businesses in addressing sustainability, these factions couldn’t be further apart. Sadly, over time, these groups have become suspicious and distrustful of each other. Like those among members of a family, the fights among these closely related groups are sometimes vicious. Most days, it’s hard to even imagine we all think of ourselves as “environmentalists.”

A theory of change is simply that: a theory about how you think the world changes for the better.

Yet we all generally want the same things: a sustainable world, where people can live full and productive lives without compromising the environment or the opportunities of future generations. So why all the friction? Why all the division, debate and suspicion?

I think a lot these differences boil down to people having different “theories of change.”

A theory of change is simply that: a theory about how you think the world changes for the better. Some people, like Bill McKibben, think the world changes because of political and social movements. Others, like the Deep Ecologists, think it takes fundamental cultural shifts in our attitudes toward nature, while Neo-environmentalists think it takes economic transformation and radical technological change. And there are many other theories of change, each representing a deeply held set of assumptions about how the world changes.

Regardless of their underlying assumptions, the most important thing about a theory of change is that it be a real, working theory. To me, as a scientist, this means it starts as a hypothesis (an initial guess or assumption) and then is constantly tested against data through ongoing observation and experimentation. This is how a theory works: It must be tested. Data, not our feelings, rule. If the data say a theory is wrong, then it’s simply wrong — and it’s time to move on to a new one.

But most of us are reluctant to give up our deeply held assumptions, and are unwilling to put our theory of change to the test. We stubbornly ignore the data around us and cling to these theories even in the face of contradictory evidence.

Let’s consider two different theories of change that are pretty common these days.

First, many of my friends are deeply concerned about climate change and have become involved with Bill McKibben’s latest political movement, focused on stopping the construction of the Keystone XL tar sands pipeline. It’s easy to admire these folks: They’re committed and deeply caring, and are putting themselves on the front line. When challenged on the efficacy of their movement — especially by those who say that Keystone would only add a tiny amount to the nation’s emissions, and that reducing coal use, improving efficiency and deploying more renewables would have far bigger impacts — they speak of the powerful symbolism of the pipeline, and its ability to galvanize a larger climate “movement” in the future. That makes sense, but when was the last time a social or political movement caused a major environmental policy change in Washington? Wasn’t it the early 1970s when we saw landmark federal legislation to protect clean air, clean water and endangered species? (If I’m wrong about this, please let me know.) In today’s toxic political landscape, where outside money, dime-a-dozen pundits and hyper-partisanship have utterly gridlocked D.C., do we honestly think activists and protests can change anything? With all possible respect, I have to ask: Where is the evidence to support this theory of change? (For the record, I’ll be very happy to be proven wrong about this. But, for the moment, I just don’t see enough data to support this theory. Instead, I see people taking this as an article of faith more than an observable fact.)