Lyric discussion by cossar 

Revenga is right. the lyrics are from The Screwtape Letters. Here's the section of the book where Brendan got it from:

"Nothing is very strong: strong enough to steal away a man's best years not in sweet sins but in a dreary flickering of the mind over it knows not what and knows not why, in the gratification of curiosities so feeble that the man is only half aware of them, in a drumming of fingers and kicking of heels, in whistling tunes that he does not like, or in the long, dim labyrinth of reveries that have not even lust or ambition to give them a relish, but which, once chance association has started them, the creature is too weak and fuddled to shake off. You will say that these are very small sins; and doubtless, like all young tempters, you are anxious to be able to report spectacular wickedness. But do remember, the only thing that matters is the extent to which you separate the man from the Enemy. It does not matter how small the sins are, provided that their cumulative effect is to edge the man away from the Light and out into the Nothing. Murder is no better than cards if cards can do the trick. Indeed the safest road to Hell is the gradual one - the gentle slope, soft underfoot, without sudden turnings, without milestones, without signposts."

A dead fire in a cold room is an example of how his character can be satisfied with nothing.

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