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"Dark Side of the Moon" is a study of how the pressures of society can lead to madness and isolation.

The "Speak to Me" intro, is the foreshadowing of the entire album. The ch-ching! of the cash register, the tick-tick-ticking of the clocks, the laughter--they all provide the mood for the album.

"Breathe" can be about two things--if we're concentrating on a character (which I prefer to follow), rather than insanity as a whole. 1.) This, plus the heartbeat from "Speak to Me," symbolizes the man's birth, and the promise that he will "race towards an early grave", a result of
2.) Overworking himself to isolation and early death.

"On the Run" symbolizes travel (its original title), and the never-ending struggle of moving from place to place. Also, the running footsteps and mocking laughter can resemble a sense of paranoia beginning to develop within the man's psyche.

"Time"'s meaning is, to put it in brief terms, the sheer pain of knowing we can't control time, there is never enough, and our lives seem to revolve around time and its tyrannic presence.

The reprise of "Breathe" is about the central character's increasing dependence on isolation from his worries. Some lyrics also reflect Roger Waters' (principle lyricist) belief that organized structures of power (like certain church factions) can also be a great pressure.
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A brave man once requested me, to answer questions that are key. "Is it to be or not to be?" And I replied, "Oh, why I ask me?"