The Latin verb influo (“flow in”) gives us influence, influx, influenza and Influo, a Belgian “influencer marketing platform.” At first it was mostly used to talk about water, but by at least the third century A.D., early Christians were using influo to describe the Holy Spirit flowing into the souls of believers. In the Middle Ages, a noun form emerged — “influentia.” Its meaning was, essentially, astrological: Planets were thought to pour their influences down on Earth, spreading things like benevolence (Jupiter), conflict (Mars) and madness (the moon — hence “lunacy”). “What euill starre/On you hath fround, and pourd his influence bad?” asked the Tudor poet Edmund Spenser in 1590.

Along with this belief came a certain anxiety about its spiritual implications. The medieval Christian theologian Thomas Aquinas accepted that the planets and stars could affect our physical bodies, but he rejected the idea that they could control our minds, which would mean the end of free will. C.S. Lewis summed up medieval attitudes in his 1964 book “The Discarded Image”: The belief, he wrote, was that influence “generates, not a necessity, but only a propensity, to act thus or thus.” And this propensity could be resisted: “The wise man will over-rule the stars. But more often it will not be resisted, for most men are not wise.”

Over time, as astrology was discarded as junk science, influence gradually became the work not of celestial bodies but of governments and advertisers. Propaganda became easier with the printing press; influence became a bona fide business with the rise of behavioral marketing. Lewis complained that this 20th-century use of “influence” had a “withered senility” in comparison to its powerful original meaning. But that astrological sense of influence as a malign, opaque and distant force — something that tempts us against our characters or better wisdom — still lingers in the way people talk about the dark orbits of global power and wealth. Just as kings were once thought to rise and fall by the power of the heavens, we now wonder which influence broker might have been responsible for a president’s election: the Russians or the Mercers?

For much of the 20th century, America’s dark planet was the Soviet Union. The Soviet brain, said the director of central intelligence Allen Dulles in a 1953 speech, is “a phonograph playing a disc put on its spindle by an outside genius over which it has no control.” Soviet brainwashing, he said, might come through indoctrination or some kind of “lie serum,” but whatever it was, it was “abhorrent.” (Never mind that, the same year, Dulles started a program to conduct secret mind-control experiments on unsuspecting Americans and Canadians.) At least, he said, the Soviets didn’t have the manpower to do it on a grand scale.