Lyric discussion by Scott guion 

Cover art for Amphetamine Logic lyrics by Sisters of Mercy, The

(I know this isnt what this song interprets)

But I like to think that the "Knife" means a Amphetamine pill. "One Life" is the feeling he gets from the drug.

He says, "Nothing but the knife to live for One life all I need." Everytime he pops a pill.

By the end of the song he becomes so addicted you hear his voice change from calm to almost eratic.

This is how I felt while on Amphetamine.

@Scott guion You're very close in saying that the knife means a speed pill. At the time of writing this song, Eldritch was taking heavy doses of amphetamine in powder form off a knife blade.