This one has several: I like the note in the introduction.
Muir used the phrase "indoor philosophy" to explain why Bostonians in the company of Emerson refused to let the old man join in one of Muir's wild treks.
"He seemed as serene as a sequoia, his head in the empyrean; and forgetting his age, plans, duties, ties of every sort, I proposed an immeasurable camping trip back in the heart of the mountains. He seemed anxious to go, but considerately mentioned his party. I said: "Never mind. The mountains are calling; run away, and let plans and parties and dragging lowland duties all gang tapsal-teerie. We'll go up a cañon singing your own song, "Good-by, proud world! I'm going home, in divine earnest. Up there lies a new heaven and a new earth; let us go to the show." But alas, it was too late,—too near the sundown of his life. The shadows were growing long, and he leaned on his friends. His party, full of indoor philosophy, failed to see the natural beauty and fullness of promise of my wild plan, and laughed at it in good-natured ignorance, as if it were necessarily amusing to imagine that Boston people might be led to accept Sierra manifestations of God at the price of rough camping."
Abbey took the phrase and used it as a weapon. "In metaphysics, the notion that earth and all that's on it is a mental construct is the product of people who spend their lives inside rooms," he said. "It is an indoor philosophy."
But don't stop reading there. Keep going.