“Mobs have passions, not brains.”

The Saviors of God (1923)
Context: You are not a miserable and momentary body; behind your fleeting mask of clay, a thousand-year-old face lies in ambush. Your passions and your thoughts are older than your heart or brain.

Thunder on the Right
Don Camillo and the Prodigal Sun (1952)

Them Belly Full (But We Hungry), from the album Natty Dread (1974)
Song lyrics

Adams as portrayed in the HBO Miniseries John Adams (2008); this has sometimes been cited as having been actually said or written by the historical John Adams.
Misattributed

Source: The Curse of Lono

John Perry Barlow 2.0 (2004)
Context: You now have two distinct ways of gathering information beyond what you yourself can experience. One of them is less a medium than an environment — the Internet — with a huge multiplicity of points of view, lots of different ways to find out what's going on in the world. Lots of people are tuned to that, and a million points of view have bloomed. It creates a cacophony of viewpoints that doesn't have any political coherence at all, a beautiful melee, but it doesn't have the capacity to create large blocs of belief.
The other medium, TV, has a much smaller share of viewers than at any time in the past, but those viewers get all their information there. They get turned into a very uniform belief block. TV in America created the most coherent reality distortion field that I’ve ever seen. Therein is the problem: People who vote watch TV, and they are hallucinating like a sonofabitch. Basically, what we have in this country is government by hallucinating mob.

Source: Queen's Gambit Declined (1989), Chapter 2 (p. 25)

"Quotes", The Educated Imagination (1963), Talk 6: The Vocation of Eloquence