This quotation comes from Sagan's Cosmos series, Episode 3: The Harmony of the Worlds, and the "he" refers to Johannes Kepler. Kepler nursed a long and deep belief that planetary motion in our solar system was somehow connected to the five Platonic solids.* He worked on creating a model of this idea for a very long time. It was his passion and solitary purpose...until he hit the wall of reality, which didn't match his belief. He graciously let it go.
"In Mercerism, an absolute evil plucked at the threadbare cloak of the tottering, ascending old man, but it was never clear who or what this evil presence was. A Mercerite sensed evil without understanding it. Put another way, a Mercerite was free to locate the nebulous presence of The Killers wherever he saw fit." --From Philip K. Dick's Do Androids Dream of Electric Sheep?
Thank goodness there really are no Mercerites in the U.S.**
Also from Androids, but Dick was quoting Mozart's The Magic Flute:
"Papageno: My child, what should we now say?
And finally, the quotation under my profile photo is also from Androids. But I could have written it about you, dear readers. I would score terribly high to view such a horror.***Pamina: The truth. That's what we will say."
*Could Mitt Romney be the sixth? (Bazinga).
***No "Bazinga." A wink and a smile.