From Wikipedia:
Roger Waters has stated that the insanity-themed lyrics are based on former Floyd frontman Syd Barrett's mental instability, with the line "I'll see you on the dark side of the moon" indicating that Waters felt that he related to Barrett in terms of mental idiosyncrasies. The line "And if the band you're in starts playing different tunes..." is referential to Syd Barrett's behaviour towards the end of his tenure with the band; because of his mental problems, there were more than a few occasions where Barrett would play a different song than the rest of the band in the middle of a performance. The song features a rather famous opening line, "The lunatic is on the grass...", whereby Waters is referring to areas of turf which display signs saying "Please keep off the grass" with the exaggerated implication that the disobeying of such signs might be an indication of insanity. The tongue in cheek nature of the lyrics is further emphasized by Waters' assertion in the 2003 documentary "Classic Albums: The Making of The Dark Side of the Moon" that not letting people on such beautiful grass was the real insanity. The line, "Got to keep the loonies on the path" supports this, suggesting the social rule that people must not walk off the path and step onto the grass, but conform to the standards of normal 'sane' society.
Incidentally, Waters has said that the particular patch of grass he had in mind when writing was to the rear of Kings College, Cambridge.
The German literary scholar and media theorist Friedrich Kittler attaches great relevance to the song, referring to its lyrics as well as to its technological arrangement. For him, the three verses stage the (sound) technological evolution from mono to stereo, culminating in total, "maddening" surround sound.
Lyrics:
The lunatic is on the grass.
The lunatic is on the grass.
Remembering games and daisy chains and laughs.
Got to keep the loonies on the path.
The lunatic is in the hall.
The lunatics are in my hall.
The paper holds their folded faces to the floor
And every day the paper boy brings more.
And if the dam breaks open many years too soon
And if there is no room upon the hill
And if your head explodes with dark forebodings too
I'll see you on the dark side of the moon.
The lunatic is in my head.
The lunatic is in my head
You raise the blade, you make the change
You re-arrange me 'til I'm sane.
You lock the door
And throw away the key
There's someone in my head but it's not me.
And if the cloud bursts, thunder in your ear
You shout and no one seems to hear.
And if the band you're in starts playing different tunes
I'll see you on the dark side of the moon.
