I love Neil de Grasse Tyson. He’s funny, he’s smart, and he makes science understandable and exciting.

He recently tweeted something that has had nearly 10,000 retweets and 20,000 likes, and on first glance, sounds fantastic: “Earth needs a virtual country: #Rationalia, with a one-line Constitution: All policy shall be based on the weight of evidence.”

Especially in this particular election year, with its debased rhetoric and litany of false statements, “Rationalia” might sound like a great place to live. No more lies! No more ignorance! No more blind ideology!

There’s only one problem: evidence can’t dictate policy. Science and rationality are great for knowing what is. But if we want to know what to do, we need more.

Let’s say citizens of Rationalia wanted to address an issue such as education reform, something citizens of Rationalia would clearly care about. How will we know we succeeded? “Weight of evidence” requires numbers, so that would mean some kind of testing. But what should we test? The typical subjects like math, history or English, or newer assessments like grit and joy? At what point do we use the assessment — the year end? 5 years out? 25 years out? And let’s not even get into the debate about whether test scores are a good measure of educational success at all!

Evidence is great, but when it comes to policy, it has be gauged against what we want to have happen. We need to start by articulating the values we hold most dear, and only then should we see if the evidence tells us if we are getting closer to or farther away from our goal.

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