“The mythos. The mythos is insane. That's what he believed. The mythos that says the forms of this world are real but the Quality of this world is unreal, that is insane!”

Source: Zen and the Art of Motorcycle Maintenance (1974), Ch. 28
Context: The mythos-over-logos argument points to the fact that each child is born as ignorant as any caveman. What keeps the world from reverting to the Neandertal with each generation is the continuing, ongoing mythos, transformed into logos but still mythos, the huge body of common knowledge that unites our minds as cells are united in the body of man. To feel that one is not so united, that one can accept or discard this mythos as one pleases, is not to understand what the mythos is.

Source: Zen and the Art of Motorcycle Maintenance (1974), Ch. 29
Context: What is essential to understand at this point is that until now there was no such thing as mind and matter, subject and object, form and substance. Those divisions are just dialectical inventions that came later. The modern mind sometimes tends to balk at the thought of these dichotomies being inventions and says, "Well, the divisions were there for the Greeks to discover," and you have to say, "Where were they? Point to them!" And the modern mind gets a little confused and wonders what this is all about anyway, and still believes the divisions were there.
But they weren't, as Phædrus said. They are just ghosts, immortal gods of the modern mythos which appear to us to be real because we are in that mythos. But in reality they are just as much an artistic creation as the anthropomorphic Gods they replaced.

Source: Zen and the Art of Motorcycle Maintenance (1974), Ch. 28
Context: Religion isn't invented by man. Men are invented by religion. Men invent responses to Quality, and among these responses is an understanding of what they themselves are. You know something and then the Quality stimulus hits and then you try to define the Quality stimulus, but to define it all you've got to work with is what you know. So your definition is made up of what you know. It's an analogue to what you already know. It has to be. It can't be anything else. And the mythos grows this way. By analogies to what is known before. The mythos is a building of analogues upon analogues upon analogues. These fill the collective consciousness of all communicating mankind. Every last bit of it. The Quality is the track that directs the train. What is outside the train, to either side—that is the terra incognita of the insane. He knew that to understand Quality he would have to leave the mythos. That's why he felt that slippage. He knew something was about to happen.

As quoted in Wisdom for the Soul : Five Millennia of Prescriptions for Spiritual Healing (2006) by Larry Chang, p. 412; this might be a paraphrase, as the earliest occurrence of this phrase thus far located is in the form: "Ronald David Laing has shocked many people when he suggested in 1972 that insanity can be a perfectly rational adjustment to an insane world." in Studii de literatură română și comparată (1984), by The Faculty of Philology-History at Universitatea din Timișoara. A clear citation to Laing's own work has not yet been found.
Disputed

Cleric: US threats, fruits of insanity, Press TV, October 2007 http://www.presstv.ir/detail.aspx?id=28077&sectionid=351020203,

Grey Eminence (1940)

The Stage, January 2007 https://www.thestage.co.uk/opinion/2014/remembering-rik-mayall-one-oddest-fun-interviews-ive-ever-done/

2000s, Kingdom of Fear: Loathsome Secrets of a Star-crossed Child in the Final Days of the American Century (2004)