… is from page 118 of H.L. Mencken’s, A Second Mencken Chrestomathy  (1995); specifically, it’s reprinted from a piece that Mencken wrote for the June 1920 Smart Set:
It is not materialism that is the chief curse of the world, but idealism. Men get into trouble by taking all their gaudy visions and hallucinations seriously.
I would amend the above nugget of indispensable wisdom only by pointing out that there is good idealism, one that projects no gaudy visions or hallucinations because it is grounded in realism about human nature and, hence, recognizes the limits of the human mind, the weakness of the human will, and the impossibility of human beings to care for strangers as deeply as they care for themselves, their families, and their close friends – namely, the idealism that yearns for a world in which power is decentralized sufficiently to allow individuals to lead their lives as they choose, rather than along lines enforced by some Leader or collective – the realism that sees all calls for ‘great leaders’ as insanity, and all belief that collectives have wills as baseless and dangerous anthropomorphism – the idealism that springs from an appreciation for, even a celebration of, bourgeois virtues  – the idealism whose champions understand that social engineering (either writ large, as in Soviet Russia and Maoist China, or writ smaller, as in Obamacare and minimum-wage legislation) is stupidity and evil masquerading as wisdom and goodness – the idealism that understands that true peace (as opposed to mere cease-fires) is best assured by commerce and toleration rather than by guns and diktats – the idealism that rejects nationality as defining who ‘we’ are and, instead, judges and treasures everyone according to the content of their character and not by the color of their passports – the idealism that seeks maximum scope for capitalist acts among consenting adults and (hence) ever-shrinking scope for political demands by the politically powerful upon the politically impotent. In short, it is an idealism that rejects any and all forms of collective or social engineering, regardless of the actual or ostensible motives – and of the school names on the diplomas – of the wannabe engineers.