I'm managed to find the source for this quote by Sol LeWitt:
The system is the work of art; the visual work of art is the proof of the system. The visual aspect can't be understood without understanding the system. It isn't what it looks like but what it is that is of basic importance.
The original source would appear to be an interview published in BOMB at www.bombsite.com/issues/85/articles/2583.
It's a great quote and a very interesting interview in general.
Author: Sean Clark