Carl Sagan, one of the world’s most beloved astronomers and science educators, would have been 81 today. In honor of his birthday, here are five pieces of advice the sage explainer of the cosmos dished out while he was still alive: 

1. “ANYTHING THAT'S TRULY REAL CAN STAND UP TO SCRUTINY.”

In an essay for New York magazine in 2014, Sagan’s daughter, Sasha, wrote about asking her father whether he thought he’d see his late parents in the afterlife:

[H]e told me, very tenderly, that it can be dangerous to believe things just because you want them to be true. You can get tricked if you don’t question yourself and others, especially people in a position of authority. He told me that anything that’s truly real can stand up to scrutiny.

2. INCLUSION IS GOOD FOR EVERYONE. 

In 1981, Sagan wrote a letter protesting The Explorers Club’s policy against admitting women as members. “Today women are making extraordinary contributions in areas of fundamental interest to our organization,” he argued. “They are of true historical significance. If membership in The Explorers Club is restricted to men, the loss will be ours; we will only be depriving ourselves.”

3. EMBRACE CHANGE, AND PLAN FOR IT. 

“Accommodation to change, the thoughtful pursuit of alternative futures, is the key to the survival of civilization and perhaps of humanity,” Sagan wrote in an essay on science fiction’s role in society in 1978.

4. BE SKEPTICAL OF THE RESULTS YOU'RE MOST EXCITED ABOUT.

I personally have been captured by the notion of extraterrestrial life, and especially extraterrestrial intelligence, from childhood. It swept me up,” Sagan told NOVA in an interview for the program Kidnapped by UFOs? in 1996. “It would be an absolutely transforming event in human history [to find extraterrestrial life]. But, the stakes are so high on whether it's true or false that we must demand the more rigorous standards of evidence—precisely because it's so exciting. That's the circumstance in which our hopes may dominate our skeptical scrutiny of the data.”

5. TAKE CARE OF THE EARTH, AND EACH OTHER.

“We must stop pretending we’re something we are not,” Sagan and his wife and collaborator, Ann Druyen, wrote in the book Shadows of Forgotten Ancestors:

"Somewhere between romantic, uncritical anthropomorphizing of the animals and the anxious, obdurate refusal to recognize our kinship with them—the latter made tellingly clear in the still-widespread notion of 'special' creation—there is a broad middle ground on which we humans can take our stand … In accord with the unforgiving principle of natural selection, we are charged with our own preservation—under penalty of extinction."