PI is a variable.

http://answers.yahoo.com/question/index?qid=20080708162347AAh4RJ4
A variable is an unknown

but no one ever calculated or seen value of Pi, so Pi must be unknown. Besides, even on different SBCL implementations Pi has different values.

π is a constant, not a variable. If it were a variable you could take a derivative with respect to it, which you can't.

π is not unknown, just irrational, which means it can't be expressed in bases such as base-10. In base-π, π would 10, and 1 would be irrational, but that doesn't mean 1 is unknown.